[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>

net/ipv6/ipv6_sockglue.c

@@ -287,7 +287,7 @@ int ipv6_setsockopt(struct sock *sk, int level, int optname,
 	{
 		struct ipv6_txoptions *opt;
 		if (optlen == 0)
-			optval = 0;
+			optval = NULL;
 
 		/* hop-by-hop / destination options are privileged option */
 		retv = -EPERM;