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

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.

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+4ece1a28b8f4730547c9@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>