Bug report: crash on GIF file

[M-Gonzalo]

Pretty self-explanatory. I just found a tiny file that makes precomp crash on both Linux x86-64 and ARM. Didn't test on Windows or Mac though.

"bug785973.gif" is the original filename and it appears to be a crafted test file from the "gdk-pixbuf2-tests" package. Just out of curiosity I also tested precomp on the rest of the contents of the package and I did find two edge cases of huge images that are not correctly processed. See #117 and #118.