cfg80211: fix NULL ptr deref

commit 211a4d12abf86fe0df4cd68fc6327cbb58f56f81
  Author: Johannes Berg <johannes@sipsolutions.net>
  Date:   Tue Oct 20 15:08:53 2009 +0900

      cfg80211: sme: deauthenticate on assoc failure

introduced a potential NULL pointer dereference that
some people have been hitting for some reason -- the
params.bssid pointer is not guaranteed to be non-NULL
for what seems to be a race between various ways of
reaching the same thing.

While I'm trying to analyse the problem more let's
first fix the crash. I think the real fix may be to
avoid doing _anything_ if it ended up being NULL, but
right now I'm not sure yet.

I think
http://bugzilla.kernel.org/show_bug.cgi?id=14342
might also be this issue.

Reported-by: Parag Warudkar <parag.lkml@gmail.com>
Tested-by: Parag Warudkar <parag.lkml@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>