mac80211: check vif pointer before airtime calculation

In case of monitor mode injection, vif may be NULL, don't crash
on that in ieee80211_calc_expected_tx_airtime().
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20200221104544.dddb7a3568fd.I0ede2733a3c76e95daeab07538449ea847e7b78d@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -5,7 +5,7 @@
  * Copyright 2006-2007	Jiri Benc <jbenc@suse.cz>
  * Copyright 2007	Johannes Berg <johannes@sipsolutions.net>
  * Copyright 2013-2014  Intel Mobile Communications GmbH
- * Copyright (C) 2018 Intel Corporation
+ * Copyright (C) 2018-2020 Intel Corporation
  *
  * Transmit and frame generation functions.
  */
@@ -3682,7 +3682,8 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
 encap_out:
 	IEEE80211_SKB_CB(skb)->control.vif = vif;
 
-	if (wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL)) {
+	if (vif &&
+	    wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL)) {
 		u32 airtime;
 
 		airtime = ieee80211_calc_expected_tx_airtime(hw, vif, txq->sta,