ImageMagick 7.0.8-35 has a memory leak in coders/dot.c, as demonstrated by AcquireMagickMemory in MagickCore/memory.c.
### 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
memory leaks in AcquireMagickMemory
```
=================================================================
==54549==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 13504 byte(s) in 1 object(s) allocated from:
    #0 0x7f1d5e55d602 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x98602)
    #1 0x43f173 in AcquireMagickMemory MagickCore/memory.c:478
    #2 0x412e4f in AcquireCriticalMemory MagickCore/memory-private.h:64
    #3 0x4131c2 in AcquireImage MagickCore/image.c:171
    #4 0x58026c in ReadDOTImage coders/dot.c:129
    #5 0x805aeb in ReadImage MagickCore/constitute.c:547
    #6 0x807c95 in ReadImages MagickCore/constitute.c:917
    #7 0xad7412 in CompositeImageCommand MagickWand/composite.c:534
    #8 0xc9c5ea in MagickCommandGenesis MagickWand/mogrify.c:184
    #9 0x40e941 in MagickMain utilities/magick.c:149
    #10 0x40eb22 in main utilities/magick.c:180
    #11 0x7f1d5a84d82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
```
### Steps to Reproduce
./magick composite input1 input2 output.aai

### System Configuration
1、Linux ubuntu 4.15.0-42-generic #45~16.04.1-Ubuntu SMP Mon Nov 19 13:02:27 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
2、./configure CC="gcc" CXX="g++" CFLAGS="-g -fsanitize=address"
- ImageMagick version:7.0.8-35


<!-- Thanks for reporting the issue to ImageMagick! -->
[memory_leaks.zip](https://github.com/ImageMagick/ImageMagick/files/3002595/memory_leaks.zip)

credit:
  ADlab of venustech


memory leaks
This is a GitHub Issue
repo:ImageMagick
owner:ImageMagick
Title : memory leaks
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
memory leaks in AcquireMagickMemory
```
=================================================================
==54549==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 13504 byte(s) in 1 object(s) allocated from:
    #0 0x7f1d5e55d602 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x98602)
    #1 0x43f173 in AcquireMagickMemory MagickCore/memory.c:478
    #2 0x412e4f in AcquireCriticalMemory MagickCore/memory-private.h:64
    #3 0x4131c2 in AcquireImage MagickCore/image.c:171
    #4 0x58026c in ReadDOTImage coders/dot.c:129
    #5 0x805aeb in ReadImage MagickCore/constitute.c:547
    #6 0x807c95 in ReadImages MagickCore/constitute.c:917
    #7 0xad7412 in CompositeImageCommand MagickWand/composite.c:534
    #8 0xc9c5ea in MagickCommandGenesis MagickWand/mogrify.c:184
    #9 0x40e941 in MagickMain utilities/magick.c:149
    #10 0x40eb22 in main utilities/magick.c:180
    #11 0x7f1d5a84d82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
```
### Steps to Reproduce
./magick composite input1 input2 output.aai

### System Configuration
1、Linux ubuntu 4.15.0-42-generic #45~16.04.1-Ubuntu SMP Mon Nov 19 13:02:27 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
2、./configure CC="gcc" CXX="g++" CFLAGS="-g -fsanitize=address"
- ImageMagick version:7.0.8-35


<!-- Thanks for reporting the issue to ImageMagick! -->
[memory_leaks.zip](https://github.com/ImageMagick/ImageMagick/files/3002595/memory_leaks.zip)

credit:
  ADlab of venustech


--- end body ---
