mac80211: fix issue with possible txq NULL pointer

Drivers that do not have the BUFF_MMPDU_TXQ flag set will not have a
TXQ for the special TID = 16.

In this case, the last member in the *struct ieee80211_sta* txq array
will be NULL.

We must check this in order not to get a NULL pointer dereference when
iterating the txq array.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/util.c

@@ -264,6 +264,9 @@ static void __ieee80211_wake_txqs(struct ieee80211_sub_if_data *sdata, int ac)
 		for (i = 0; i < ARRAY_SIZE(sta->sta.txq); i++) {
 			struct ieee80211_txq *txq = sta->sta.txq[i];
 
+			if (!txq)
+				continue;
+
 			txqi = to_txq_info(txq);
 
 			if (ac != txq->ac)