[IPV6]: Fix for ipv6_setsockopt NULL dereference

I came across this bug in http://bugzilla.kernel.org/show_bug.cgi?id=8155

Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>