Magick's failure to limit MVG mutual references forming a loop

ImageMagick is free and open-source software used for editing and manipulating digital images. Prior to version 7.1.2-12, Magick fails to check for circular references between two MVGs, leading to a stack overflow. This is a DoS vulnerability, and any situation that allows reading the mvg file will be affected. Version 7.1.2-12 fixes the issue.