ImageMagick has memory leak of watermark Image object in ReadSTEGANOImage on multiple error/early-return paths

Summary

In ReadSTEGANOImage() (coders/stegano.c), the watermark Image object is not freed on three early-return paths, resulting in a definite memory leak (~13.5KB+ per invocation) that can be exploited for denial of service.

Direct leak of 13512 byte(s) in 1 object(s) allocated from:
    #0 0x7f5c11e27887 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x55cdc38f65c4 in AcquireMagickMemory MagickCore/memory.c:536
    #2 0x55cdc38f65eb in AcquireCriticalMemory MagickCore/memory.c:612
    #3 0x55cdc3899e91 in AcquireImage MagickCore/image.c:154