wifi: mac80211: potential NULL dereference in ieee80211_tx_control_port()

The ieee80211_lookup_ra_sta() function will sometimes set "sta" to NULL
so add this NULL check to prevent an Oops.

Fixes: 9dd19538 ("wifi: nl80211/mac80211: clarify link ID in control port TX")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YuKcTAyO94YOy0Bu@kiliSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -5900,7 +5900,7 @@ int ieee80211_tx_control_port(struct wiphy *wiphy, struct net_device *dev,
 		 * for MLO STA, the SA should be the AP MLD address, but
 		 * the link ID has been selected already
 		 */
-		if (sta->sta.mlo)
+		if (sta && sta->sta.mlo)
 			memcpy(ehdr->h_source, sdata->vif.addr, ETH_ALEN);
 	}
 	rcu_read_unlock();