mac80211: Fix null pointer dereference with iTXQ support

This change adds null pointer check before dereferencing pointer dev on
netif_tx_start_all_queues() when an interface is added.
With iTXQ support, netif_tx_start_all_queues() is always called while
an interface is added. however, the netdev queues are not associated
and dev is null when the interface is either NL80211_IFTYPE_P2P_DEVICE
or NL80211_IFTYPE_NAN.
Signed-off-by: Chunho Lee <ch.lee@newracom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -731,6 +731,7 @@ int ieee80211_do_open(struct wireless_dev *wdev, bool coming_up)
 	    sdata->vif.type == NL80211_IFTYPE_AP_VLAN ||
 	    local->ops->wake_tx_queue) {
 		/* XXX: for AP_VLAN, actually track AP queues */
+		if (dev)
 			netif_tx_start_all_queues(dev);
 	} else if (dev) {
 		unsigned long flags;