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