mac80211: fix potential NULL dereference

Smatch complains that we could dereference skb later in the function.
It's probably unlikely, but we may as well return here and avoid it.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[change summary]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -49,7 +49,7 @@ static struct sk_buff *remove_monitor_info(struct ieee80211_local *local,
 			/* driver bug */
 			WARN_ON(1);
 			dev_kfree_skb(skb);
-			skb = NULL;
+			return NULL;
 		}
 	}