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

net/ipv6/ipv6_sockglue.c

···
 		}
 
 		/* routing header option needs extra check */
-		if (optname == IPV6_RTHDR && opt->srcrt) {
+		if (optname == IPV6_RTHDR && opt && opt->srcrt) {
 			struct ipv6_rt_hdr *rthdr = opt->srcrt;
 			switch (rthdr->type) {
 			case IPV6_SRCRT_TYPE_0: