libgd 2.2.5
In gd_gif_out::gdImageGifAnimAddCtx we find this comment:

            /* create optimized animation.  Compare this image to
               the previous image and crop the temporary copy of
               current image to include only changed rectangular
      [snip]
              Images should be of same size.  If not, a temporary
               copy is made with the same size as previous image.
             */

The "should be" remark is true, but in fact no such copy is made.
As a result, whenever (tim->sy > prev_tim->sy) the code segfaults while trying to compare pixels.

De-selecting optimization avoids the bad code and image generation proceeds normally.
The problem was found while tracing a segfault generated by calls into libgd from gnuplot.
https://sourceforge.net/p/gnuplot/bugs/1992/
It is 100% reproducible on multiple platforms, but I have not tried to construct a small artificial demonstration code not involving gnuplot.