Fix a wpg file out of bound
Origin: upstream,
https://github.com/ImageMagick/ImageMagick/commit/bef1e4f637d8f665bc133a9c6d30df08d983bc3a
bug: https://github.com/ImageMagick/ImageMagick/issues/120
bug-ubuntu:
https://bugs.launchpad.net/ubuntu/+source/imagemagick/+bug/1542114
bug-debian: https://bugs.debian.org/823750
bug: https://github.com/ImageMagick/ImageMagick/issues/120
