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

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

commit 864a6040f395464003af8dd0d8ca86fed19866d4 upstream.

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.

Fixes: 3f52b7e328c5 ("mac80211: mesh power save basics")
Fixes: ce662b44ce22 ("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>
[bwh: Backported to 3.2: Drop change to mps_qos_null_get()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>