ImageMagick 7.0.8-50 Q16 has a heap-based buffer over-read at MagickCore/pixel-accessor.h in GetPixelChannel.
### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have verified that I am using the latest version of ImageMagick
- [x] I have searched [open](https://github.com/ImageMagick/ImageMagick/issues) and [closed](https://github.com/ImageMagick/ImageMagick/issues?q=is%3Aissue+is%3Aclosed) issues to ensure it has not already been reported

### Description
<!-- A description of the bug or feature -->
There's a heap-buffer-overflow at MagickCore/pixel-accessor.h:116:10 in GetPixelChannel.

### Steps to Reproduce
<!-- List of steps, sample code, failing test or link to a project that reproduces the behavior.
     Make sure you place a stack trace inside a code (```) block to avoid linking unrelated issues -->
run_cmd:
`magick -seed 0 "(" magick:netscape -monochrome  ")" "(" magick:netscape +repage ")" -geometry 433%-80-57 -adjoin -evaluate-sequence Median   tmp`

Here's ASAN log.
```
==30168==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7febb7ba1400 at pc 0x7febc5808632 bp 0x7ffd269baff0 sp 0x7ffd269bafe8
READ of size 4 at 0x7febb7ba1400 thread T0
    #0 0x7febc5808631 in GetPixelChannel ./MagickCore/pixel-accessor.h:116:10
    #1 0x7febc5805ff6 in EvaluateImages MagickCore/statistic.c:587:33
    #2 0x7febc4e1a5bf in CLIListOperatorImages MagickWand/operation.c:4084:22
    #3 0x7febc4e2435e in CLIOption MagickWand/operation.c:5279:14
    #4 0x7febc4c65a99 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #5 0x7febc4c66d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #6 0x7febc4cb0ba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #7 0x526f95 in MagickMain utilities/magick.c:149:10
    #8 0x5268e1 in main utilities/magick.c:180:10
    #9 0x7febbf727b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #10 0x41b069 in _start (install/bin/magick+0x41b069)

0x7febb7ba1400 is located 0 bytes to the right of 248832-byte region [0x7febb7b64800,0x7febb7ba1400)
allocated by thread T0 here:
    #0 0x4e6200 in __interceptor_posix_memalign (install/bin/magick+0x4e6200)
    #1 0x7febc569fed6 in AcquireAlignedMemory MagickCore/memory.c:265:7
    #2 0x7febc53e861c in OpenPixelCache MagickCore/cache.c:3728:46
    #3 0x7febc53ee901 in GetImagePixelCache MagickCore/cache.c:1754:18
    #4 0x7febc53f4bc9 in SyncImagePixelCache MagickCore/cache.c:5488:28
    #5 0x7febc5653831 in SetImageStorageClass MagickCore/image.c:2627:10
    #6 0x7febc54187e2 in AcquireImageColormap MagickCore/colormap.c:144:10
    #7 0x7febc575d137 in AssignImageColors MagickCore/quantize.c:514:7
    #8 0x7febc5753f38 in QuantizeImage MagickCore/quantize.c:2724:14
    #9 0x7febc53ae56c in SetImageType MagickCore/attribute.c:1495:14
    #10 0x7febc4e0cace in CLISimpleOperatorImage MagickWand/operation.c:2792:18
    #11 0x7febc4dfec78 in CLISimpleOperatorImages MagickWand/operation.c:3685:12
    #12 0x7febc4e24315 in CLIOption MagickWand/operation.c:5273:16
    #13 0x7febc4c65a99 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #14 0x7febc4c66d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #15 0x7febc4cb0ba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #16 0x526f95 in MagickMain utilities/magick.c:149:10
    #17 0x5268e1 in main utilities/magick.c:180:10
    #18 0x7febbf727b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: heap-buffer-overflow ./MagickCore/pixel-accessor.h:116:10 in GetPixelChannel
```

### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->

- ImageMagick version:
Version: ImageMagick 7.0.8-50 Q16 x86_64 2019-06-21 https://imagemagick.org

- Environment (Operating system, version and so on):
Description:	Ubuntu 18.04.1 LTS
Release:	18.04
Codename: bionic

- Additional information:
CC=clang-7 CXX=clang++-7 ./configure --disable-openmp

<!-- Thanks for reporting the issue to ImageMagick! -->

heap-buffer-overflow at MagickCore/pixel-accessor.h:116:10 in GetPixelChannel
This is a GitHub Issue
repo:ImageMagick
owner:ImageMagick
Title : heap-buffer-overflow at MagickCore/pixel-accessor.h:116:10 in GetPixelChannel
Issue date:

--- start body ---
### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have verified that I am using the latest version of ImageMagick
- [x] I have searched [open](https://github.com/ImageMagick/ImageMagick/issues) and [closed](https://github.com/ImageMagick/ImageMagick/issues?q=is%3Aissue+is%3Aclosed) issues to ensure it has not already been reported

### Description
<!-- A description of the bug or feature -->
There's a heap-buffer-overflow at MagickCore/pixel-accessor.h:116:10 in GetPixelChannel.

### Steps to Reproduce
<!-- List of steps, sample code, failing test or link to a project that reproduces the behavior.
     Make sure you place a stack trace inside a code (```) block to avoid linking unrelated issues -->
run_cmd:
`magick -seed 0 "(" magick:netscape -monochrome  ")" "(" magick:netscape +repage ")" -geometry 433%-80-57 -adjoin -evaluate-sequence Median   tmp`

Here's ASAN log.
```
==30168==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7febb7ba1400 at pc 0x7febc5808632 bp 0x7ffd269baff0 sp 0x7ffd269bafe8
READ of size 4 at 0x7febb7ba1400 thread T0
    #0 0x7febc5808631 in GetPixelChannel ./MagickCore/pixel-accessor.h:116:10
    #1 0x7febc5805ff6 in EvaluateImages MagickCore/statistic.c:587:33
    #2 0x7febc4e1a5bf in CLIListOperatorImages MagickWand/operation.c:4084:22
    #3 0x7febc4e2435e in CLIOption MagickWand/operation.c:5279:14
    #4 0x7febc4c65a99 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #5 0x7febc4c66d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #6 0x7febc4cb0ba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #7 0x526f95 in MagickMain utilities/magick.c:149:10
    #8 0x5268e1 in main utilities/magick.c:180:10
    #9 0x7febbf727b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #10 0x41b069 in _start (install/bin/magick+0x41b069)

0x7febb7ba1400 is located 0 bytes to the right of 248832-byte region [0x7febb7b64800,0x7febb7ba1400)
allocated by thread T0 here:
    #0 0x4e6200 in __interceptor_posix_memalign (install/bin/magick+0x4e6200)
    #1 0x7febc569fed6 in AcquireAlignedMemory MagickCore/memory.c:265:7
    #2 0x7febc53e861c in OpenPixelCache MagickCore/cache.c:3728:46
    #3 0x7febc53ee901 in GetImagePixelCache MagickCore/cache.c:1754:18
    #4 0x7febc53f4bc9 in SyncImagePixelCache MagickCore/cache.c:5488:28
    #5 0x7febc5653831 in SetImageStorageClass MagickCore/image.c:2627:10
    #6 0x7febc54187e2 in AcquireImageColormap MagickCore/colormap.c:144:10
    #7 0x7febc575d137 in AssignImageColors MagickCore/quantize.c:514:7
    #8 0x7febc5753f38 in QuantizeImage MagickCore/quantize.c:2724:14
    #9 0x7febc53ae56c in SetImageType MagickCore/attribute.c:1495:14
    #10 0x7febc4e0cace in CLISimpleOperatorImage MagickWand/operation.c:2792:18
    #11 0x7febc4dfec78 in CLISimpleOperatorImages MagickWand/operation.c:3685:12
    #12 0x7febc4e24315 in CLIOption MagickWand/operation.c:5273:16
    #13 0x7febc4c65a99 in ProcessCommandOptions MagickWand/magick-cli.c:477:7
    #14 0x7febc4c66d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #15 0x7febc4cb0ba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #16 0x526f95 in MagickMain utilities/magick.c:149:10
    #17 0x5268e1 in main utilities/magick.c:180:10
    #18 0x7febbf727b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: heap-buffer-overflow ./MagickCore/pixel-accessor.h:116:10 in GetPixelChannel
```

### System Configuration
<!-- Tell us about the environment where you are experiencing the bug -->

- ImageMagick version:
Version: ImageMagick 7.0.8-50 Q16 x86_64 2019-06-21 https://imagemagick.org

- Environment (Operating system, version and so on):
Description:	Ubuntu 18.04.1 LTS
Release:	18.04
Codename: bionic

- Additional information:
CC=clang-7 CXX=clang++-7 ./configure --disable-openmp

<!-- Thanks for reporting the issue to ImageMagick! -->

--- end body ---
