drm/nouveau/core: fix static checker warning

object->engine cannot be NULL, it's either valid, or an error pointer.

This particular condition shouldn't actually be possible, but just in
case, we'll keep it.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/core/object.c

@@ -295,7 +295,7 @@ nvkm_object_ctor(const struct nvkm_object_func *func,
 	INIT_LIST_HEAD(&object->head);
 	INIT_LIST_HEAD(&object->tree);
 	RB_CLEAR_NODE(&object->node);
-	WARN_ON(oclass->engine && !object->engine);
+	WARN_ON(IS_ERR(object->engine));
 }
 
 int