When we try to export a grayscale image with layers with negative offsets
to a GIH brush GIMP crashes without producing any crashlog.
Running in GDB showed us that there is heap corruption caused by incorrect
computation of buffer sizes because of the negative offsets.
In file_gih_image_to_pipe there is a comment that offsets are assumed
positive, but no checking is done whether that is correct.
Let's add some checks, set offset to 0 if negative and adjust width and
height accordingly.
(cherry picked from commit e2b1cc9476)
2a31678e9c