Resolves #11679
The loading code for ASE, ACB, and ACO palettes
set any errors with g_prefix_error (). However, the
error object is NULL at that point for those palettes.
Therefore, a NULL error object is returned to
palette_import_make_palette () which then tries to
access its attributes. This resulted in a crash.
This patch fixes the issue by using g_set_error () for
those three formats instead.
(Backport)