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