mac80211: don't return beacons when mesh is disabled

When mesh is disabled, mac80211 was returning
beacons with an empty mesh ID. That isn't
desirable, even if drivers shouldn't be trying
to get beacons to start with.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -2299,6 +2299,11 @@ struct sk_buff *ieee80211_beacon_get_tim(struct ieee80211_hw *hw,
 		struct ieee80211_mgmt *mgmt;
 		u8 *pos;
 
+#ifdef CONFIG_MAC80211_MESH
+		if (!sdata->u.mesh.mesh_id_len)
+			goto out;
+#endif
+
 		/* headroom, head length, tail length and maximum TIM length */
 		skb = dev_alloc_skb(local->tx_headroom + 400 +
 				sdata->u.mesh.vendor_ie_len);