mac80211: clear sequence/fragment number in QoS-null frames

Avoid leaking data by sending uninitialized memory and setting an
invalid (non-zero) fragment number (the sequence number is ignored
anyway) by setting the seq_ctrl field to zero.

Cc: stable@vger.kernel.org
Fixes: 3f52b7e3 ("mac80211: mesh power save basics")
Fixes: ce662b44

 ("mac80211: send (QoS) Null if no buffered frames")
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>