mac80211: don't attempt to rename ERR_PTR() debugfs dirs

mac80211: don't attempt to rename ERR_PTR() debugfs dirs

commit 517879147493a5e1df6b89a50f708f1133fcaddb upstream.

We need to dereference the directory to get its parent to
be able to rename it, so it's clearly not safe to try to
do this with ERR_PTR() pointers. Skip in this case.

It seems that this is most likely what was causing the
report by syzbot, but I'm not entirely sure as it didn't
come with a reproducer this time.

Reported-by: syzbot+4ece1a28b8f4730547c9@syzkaller.appspotmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>