autofs4: check for invalid dentry in getpath

Catch invalid dentry when calculating its path.

Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
index 1fe28e4..75e5955 100644
--- a/fs/autofs4/waitq.c
+++ b/fs/autofs4/waitq.c
@@ -171,7 +171,7 @@
 	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;
 	}