veth: fix NULL dereference in veth_dellink()

commit d0e2c55e (veth: avoid a NULL deref in veth_stats_one)
added another NULL deref in veth_dellink().

# ip link add name veth1 type veth peer name veth0
# rmmod veth

We crash because veth_dellink() is called twice, so we must
take care of NULL peer.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -426,12 +426,13 @@ static void veth_dellink(struct net_device *dev, struct list_head *head)
 	 * not being freed before one RCU grace period.
 	 */
 	RCU_INIT_POINTER(priv->peer, NULL);
+	unregister_netdevice_queue(dev, head);
 
+	if (peer) {
 		priv = netdev_priv(peer);
 		RCU_INIT_POINTER(priv->peer, NULL);
-
-	unregister_netdevice_queue(dev, head);
 		unregister_netdevice_queue(peer, head);
+	}
 }
 
 static const struct nla_policy veth_policy[VETH_INFO_MAX + 1] = {