ReadPNMImage in coders/pnm.c in GraphicsMagick 1.3.26 does not ensure the correct number of colors for the XV 332 format, leading to a NULL Pointer Dereference.
https://lists.debian.org/debian-lts-announce/2018/08/msg00002.html
https://sourceforge.net/p/graphicsmagick/bugs/465/
https://sourceforge.net/p/graphicsmagick/bugs/466/