[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>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

net/ipv6/ipv6_sockglue.c

@@ -308,7 +308,7 @@ int ipv6_setsockopt(struct sock *sk, int level, int optname,
 		}
 
 		/* 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;
 			if (rthdr->type)
 				goto sticky_done;