This image can crash both with libungif 4.1.0 and 4.1.0b1. It is special,
because it uses different Geometry and Page Geometry, see output of
identify -verbose:

Image: /home/pavel/.Emacs/Work/x1.gif
  Format: GIF (CompuServe graphics interchange format)
  Geometry: 35x47
...
  Page Geometry: 38x38+0+0

Emacs will crash in libungif, because it allocated to small region of
memory.

The same applies to next image I found in my archives: