mac80211: don't send null data packet when not associated

On resume or firmware recovery, mac80211 sends a null
data packet to see if the AP is still around and hasn't
disconnected us. However, it always does this even if
it wasn't even connected before, leading to a warning
in the new channel context code. Fix this by checking
that it's associated.

Cc: stable@vger.kernel.org
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>