mac80211: allow passing NULL to ieee80211_vif_to_wdev()

Simply return NULL in this case, instead of crashing. This can
simplify callers that would otherwise have to check for this
explicitly.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/util.c

@@ -703,7 +703,12 @@ EXPORT_SYMBOL_GPL(wdev_to_ieee80211_vif);
 
 struct wireless_dev *ieee80211_vif_to_wdev(struct ieee80211_vif *vif)
 {
-	struct ieee80211_sub_if_data *sdata = vif_to_sdata(vif);
+	struct ieee80211_sub_if_data *sdata;
+
+	if (!vif)
+		return NULL;
+
+	sdata = vif_to_sdata(vif);
 
 	if (!ieee80211_sdata_running(sdata) ||
 	    !(sdata->flags & IEEE80211_SDATA_IN_DRIVER))