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>
diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
index 9b13b8b..db343fa 100644
--- a/net/mac80211/rx.c
+++ b/net/mac80211/rx.c
@@ -49,7 +49,7 @@
 			/* driver bug */
 			WARN_ON(1);
 			dev_kfree_skb(skb);
-			skb = NULL;
+			return NULL;
 		}
 	}