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: [e-mail]
Reviewed-by: Emmanuel Grumbach <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/util.c

···
 		list_for_each_entry(sdata, &local->interfaces, list) {
 			if (sdata->vif.type != NL80211_IFTYPE_STATION)
 				continue;
+			if (!sdata->u.mgd.associated)
+				continue;
 
 			ieee80211_send_nullfunc(local, sdata, 0);
 		}