autofs4: check for invalid dentry in getpath

Catch invalid dentry when calculating its path.
Signed-off-by: NIan Kent <raven@themaw.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/waitq.c

@@ -171,7 +171,7 @@ static int autofs4_getpath(struct autofs_sb_info *sbi,
 	for (tmp = dentry ; tmp != root ; tmp = tmp->d_parent)
 		len += tmp->d_name.len + 1;
 
-	if (--len > NAME_MAX) {
+	if (!len || --len > NAME_MAX) {
 		spin_unlock(&dcache_lock);
 		return 0;
 	}