Fix a SEGV due to corrupted xwd images. CVE assignment: http://seclists.org/oss-sec/2016/q2/459 Upstream patch: https://anonscm.debian.org/cgit/collab-maint/imagemagick.git/commit/?h=debian-patches/6.8.9.9-4-for-upstream&id=82f779cbc24045af2eaecb95d0842ca7b97c71f4