Limit thread when thread limit is 0. It is a logic error that could lead to resource exhaustion. 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=2257d1eadd02d89d225fce21013a1219d221dc7d