ubifs: fix possible dereference after free

'old_idx' could be dereferenced after free via 'rb_link_node' function
call.

Fixes: b5fda08ef2 ("ubifs: Fix memleak when insert_old_idx() failed")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/tnc.c

@@ -65,6 +65,7 @@ static void do_insert_old_idx(struct ubifs_info *c,
 		else {
 			ubifs_err(c, "old idx added twice!");
 			kfree(old_idx);
+			return;
 		}
 	}
 	rb_link_node(&old_idx->rb, parent, p);