gd: apply patch for CVE-2016-3074 from upstream

Source:
https://github.com/libgd/libgd/commit/2bb97f407c1145c850416a3bfbcc8cf124e68a19.patch

The original patch contains binary data, however, which is not supported
by `patch`; we could use `git apply` here, of course, but it was simpler
to just copy-paste only the fix into a separate file and include it in
the repo.