mac80211: fix rx->key NULL pointer dereference in promiscuous mode

mac80211: fix rx->key NULL pointer dereference in promiscuous mode

Since:

commit 816c04fe7ef01dd9649f5ccfe796474db8708be5
Author: Christian Lamparter <chunkeey@googlemail.com>
Date:   Sat Apr 30 15:24:30 2011 +0200

    mac80211: consolidate MIC failure report handling

is possible to that we dereference rx->key == NULL when driver set
RX_FLAG_MMIC_STRIPPED and not RX_FLAG_IV_STRIPPED and we are in
promiscuous mode. This happen with rt73usb and rt61pci at least.

Before the commit we always check rx->key against NULL, so I assume
fix should be done in mac80211 (also mic_fail path has similar check).

References:
https://bugzilla.redhat.com/show_bug.cgi?id=769766
http://rt2x00.serialmonkey.com/pipermail/users_rt2x00.serialmonkey.com/2012-January/004395.html

Cc: stable@vger.kernel.org # 3.0+
Reported-by: Stuart D Gathman <stuart@gathman.org>
Reported-by: Kai Wohlfahrt <kai.scorpio@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>