veth: fix a NULL deref in netif_carrier_off

In commit d0e2c55e (veth: avoid a NULL deref in veth_stats_one)
we now clear the peer pointers in veth_dellink()

veth_close() must therefore make sure the peer pointer is set.
Reported-by: Tom Parkin <tom.parkin@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/veth.c

@@ -206,9 +206,11 @@ static int veth_open(struct net_device *dev)
 static int veth_close(struct net_device *dev)
 {
 	struct veth_priv *priv = netdev_priv(dev);
+	struct net_device *peer = rtnl_dereference(priv->peer);
 
 	netif_carrier_off(dev);
-	netif_carrier_off(rtnl_dereference(priv->peer));
+	if (peer)
+		netif_carrier_off(peer);
 
 	return 0;
 }