[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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/ipv6_sockglue.c

···
 	{
 		struct ipv6_txoptions *opt;
 		if (optlen == 0)
-			optval = 0;
+			optval = NULL;
 
 		/* hop-by-hop / destination options are privileged option */
 		retv = -EPERM;