ipv6: call iptunnel_xmit with NULL sock pointer if no tunnel sock is available

Fixes: 79b16aad ("udp_tunnel: Pass UDP socket down through udp_tunnel{, 6}_xmit_skb().")
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>