[Pkg-gmagick-im-team] Bug#823750: Fix an out of bound in generic decoder

[Bastien ROUCARIES]

Fix an out of bound in generic decoder

Origin: upstream,
https://github.com/ImageMagick/ImageMagick/commit/430403b0029b37decf216d57f810899cab2317dd
bug-ubuntu: https://bugs.launchpad.net/ubuntu/+source/imagemagick/+bug/1542785
bug-debian: https://bugs.debian.org/823750
bug: https://github.com/ImageMagick/ImageMagick/issues/126