Btrfs: add NULL check for do_walk_down()

btrfs_find_create_tree_block() may return NULL, so we must check the returned
value, or we will access a NULL pointer.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -5205,6 +5205,8 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 	next = btrfs_find_tree_block(root, bytenr, blocksize);
 	if (!next) {
 		next = btrfs_find_create_tree_block(root, bytenr, blocksize);
+		if (!next)
+			return -ENOMEM;
 		reada = 1;
 	}
 	btrfs_tree_lock(next);
@@ -5417,7 +5419,8 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
 		if (ret > 0) {
 			path->slots[level]++;
 			continue;
-		}
+		} else if (ret < 0)
+			return ret;
 		level = wc->level;
 	}
 	return 0;