drm/nouveau: audit bo->resource usage

Make sure we can at least move and release BOs without backing store.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220707102453.3633-4-christian.koenig@amd.com

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1008,7 +1008,8 @@ nouveau_bo_move(struct ttm_buffer_object *bo, bool evict,
 	}
 
 	/* Fake bo copy. */
-	if (old_reg->mem_type == TTM_PL_SYSTEM && !bo->ttm) {
+	if (!old_reg || (old_reg->mem_type == TTM_PL_SYSTEM &&
+			 !bo->ttm)) {
 		ttm_bo_move_null(bo, new_reg);
 		goto out;
 	}