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>