Avoid an out of bound access in viff image. 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=35aa01dd5511a2616a6427f7d5d49de0132aeb5f