imagemagick: add upstream security fix for CVE-2017-7606

imagemagick: add upstream security fix for CVE-2017-7606

This is not yet part of any release.

coders/rle.c in ImageMagick 7.0.5-4 has an "outside the range of
representable values of type unsigned char" undefined behavior issue, which
might allow remote attackers to cause a denial of service (application
crash) or possibly have unspecified other impact via a crafted image.

For more details, see:
https://blogs.gentoo.org/ago/2017/04/02/imagemagick-undefined-behavior-in-codersrle-c/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>