sit: Add missing kfree_skb() on pskb_may_pull() failure.

Noticed by Paul Marks <paul@pmarks.net>.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -596,9 +596,9 @@ static int ipip6_rcv(struct sk_buff *skb)
 	}
 
 	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);
-	kfree_skb(skb);
 	read_unlock(&ipip6_lock);
 out:
+	kfree_skb(skb);
 	return 0;
 }