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

ImageMagick is free and open-source software used for editing and manipulating digital images. Prior to versions 7.1.2-15 and 6.9.13-40, 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. Versions 7.1.2-15 and 6.9.13-40 contain a patch.