ImageMagick shouldn't have been using /bin/sh or system() in the first place. Instead it should use fork+exec or libpipeline.
http://bonedaddy.net/pabs3/log/2014/02/17/pid-preservation-s... http://libpipeline.nongnu.org/
I expect there may also be option injection vulnerabilities in the code.
http://www.defensecode.com/public/DefenseCode_Unix_WildCards...