[IPV6]: Fixes sparse warning in ipv6/ipv6_sockglue.c

The patch below fixes the following sparse warning:

net/ipv6/ipv6_sockglue.c:291:13: warning: Using plain integer as NULL pointer

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>