drm/nouveau/mem: guard against NULL pointer access in mem_del

other drivers seems to do something similar
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Cc: dri-devel <dri-devel@lists.freedesktop.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006220528.13925-2-kherbst@redhat.com

drivers/gpu/drm/nouveau/nouveau_mem.c

@@ -176,6 +176,8 @@ void
 nouveau_mem_del(struct ttm_mem_reg *reg)
 {
 	struct nouveau_mem *mem = nouveau_mem(reg);
+	if (!mem)
+		return;
 	nouveau_mem_fini(mem);
 	kfree(reg->mm_node);
 	reg->mm_node = NULL;