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

other drivers seems to do something similar

Signed-off-by: Karol Herbst <[e-mail]>
Cc: dri-devel <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: [e-mail]
Signed-off-by: Dave Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/nouveau/nouveau_mem.c

···
 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;