UBIFS: check return code of pnode_lookup

Function pnode_lookup may return ERR_PTR(...). Check for it.
Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

fs/ubifs/lpt_commit.c

@@ -705,6 +705,9 @@ static int make_tree_dirty(struct ubifs_info *c)
 	struct ubifs_pnode *pnode;
 
 	pnode = pnode_lookup(c, 0);
+	if (IS_ERR(pnode))
+		return PTR_ERR(pnode);
+
 	while (pnode) {
 		do_make_pnode_dirty(c, pnode);
 		pnode = next_pnode_to_dirty(c, pnode);