When walking backwards, we want to stop at the head sentinel, which is where scan_inst->prev->prev == NULL, not scan_inst->prev == NULL. Fixes random crashes, as well as valgrind errors. Signed-off-by: Kenneth Graunke <kenneth@whitecape.org> Reviewed-by: Matt Turner <mattst88@gmail.com> Cc: mesa-stable@lists.freedesktop.org