ImageMagick 7.0.8-50 Q16 has a stack-based buffer overflow at coders/pnm.c in WritePNMImage because of a misplaced strncpy and an off-by-one error.
### 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 stack-buffer-overflow at coders/pnm.c:1906  in WritePNMImage.

### 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 -undercolor rgb"("67,255,32")" -compress None "(" magick:rose -solarize 1%  ")" "(" magick:logo +repage ")" -size 2338x1505+3581 -print ""   tmp`

Here's ASAN log.
```
=================================================================
==25473==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe4120cba0 at pc 0x7f1d07f024d8 bp 0x7ffe412080f0 sp 0x7ffe412080e8
WRITE of size 1 at 0x7ffe4120cba0 thread T0
    #0 0x7f1d07f024d7 in WritePNMImage coders/pnm.c:1906:21
    #1 0x7f1d0779a065 in WriteImage MagickCore/constitute.c:1159:16
    #2 0x7f1d0779af8c in WriteImages MagickCore/constitute.c:1376:13
    #3 0x7f1d0715e50d in CLINoImageOperator MagickWand/operation.c:4796:14
    #4 0x7f1d071621cc in CLIOption MagickWand/operation.c:5258:7
    #5 0x7f1d06fa3f6d in ProcessCommandOptions MagickWand/magick-cli.c:529:3
    #6 0x7f1d06fa4d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #7 0x7f1d06feeba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #8 0x526f95 in MagickMain utilities/magick.c:149:10
    #9 0x5268e1 in main utilities/magick.c:180:10
    #10 0x7f1d01a65b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #11 0x41b069 in _start (install/bin/magick+0x41b069)

Address 0x7ffe4120cba0 is located in stack of thread T0 at offset 19104 in frame
    #0 0x7f1d07effdaf in WritePNMImage coders/pnm.c:1522

  This frame has 6 object(s):
    [32, 4128) 'buffer' (line 1523)
    [4256, 8352) 'magick' (line 1523)
    [8480, 12576) 'type117' (line 1675)
    [12704, 14752) 'pixels' (line 1736)
    [14880, 16928) 'pixels255' (line 1788)
    [17056, 19104) 'pixels381' (line 1857) <== Memory access at offset 19104 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow coders/pnm.c:1906:21 in WritePNMImage

```

### 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! -->

stack-buffer-overflow at coders/pnm.c:1906  in WritePNMImage 
This is a GitHub Issue
repo:ImageMagick
owner:ImageMagick
Title : stack-buffer-overflow at coders/pnm.c:1906  in WritePNMImage 
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 stack-buffer-overflow at coders/pnm.c:1906  in WritePNMImage.

### 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 -undercolor rgb"("67,255,32")" -compress None "(" magick:rose -solarize 1%  ")" "(" magick:logo +repage ")" -size 2338x1505+3581 -print ""   tmp`

Here's ASAN log.
```
=================================================================
==25473==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe4120cba0 at pc 0x7f1d07f024d8 bp 0x7ffe412080f0 sp 0x7ffe412080e8
WRITE of size 1 at 0x7ffe4120cba0 thread T0
    #0 0x7f1d07f024d7 in WritePNMImage coders/pnm.c:1906:21
    #1 0x7f1d0779a065 in WriteImage MagickCore/constitute.c:1159:16
    #2 0x7f1d0779af8c in WriteImages MagickCore/constitute.c:1376:13
    #3 0x7f1d0715e50d in CLINoImageOperator MagickWand/operation.c:4796:14
    #4 0x7f1d071621cc in CLIOption MagickWand/operation.c:5258:7
    #5 0x7f1d06fa3f6d in ProcessCommandOptions MagickWand/magick-cli.c:529:3
    #6 0x7f1d06fa4d0a in MagickImageCommand MagickWand/magick-cli.c:796:5
    #7 0x7f1d06feeba1 in MagickCommandGenesis MagickWand/mogrify.c:185:14
    #8 0x526f95 in MagickMain utilities/magick.c:149:10
    #9 0x5268e1 in main utilities/magick.c:180:10
    #10 0x7f1d01a65b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #11 0x41b069 in _start (install/bin/magick+0x41b069)

Address 0x7ffe4120cba0 is located in stack of thread T0 at offset 19104 in frame
    #0 0x7f1d07effdaf in WritePNMImage coders/pnm.c:1522

  This frame has 6 object(s):
    [32, 4128) 'buffer' (line 1523)
    [4256, 8352) 'magick' (line 1523)
    [8480, 12576) 'type117' (line 1675)
    [12704, 14752) 'pixels' (line 1736)
    [14880, 16928) 'pixels255' (line 1788)
    [17056, 19104) 'pixels381' (line 1857) <== Memory access at offset 19104 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow coders/pnm.c:1906:21 in WritePNMImage

```

### 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 ---
