mufs: set root after extending a file tree

source/fs/mufs/tree.c

@@ -113,6 +113,8 @@ int mufs_extend_tree(struct mufs *fs, struct mufs_tree *tree, uint8_t depth)
 			mufs_free_block(fs, root);
 			return -1;
 		}
+
+		tree->root = root;
 	}
 
 	return 0;