[IPV6]: Fix for ipv6_setsockopt NULL dereference

I came across this bug in http://bugzilla.kernel.org/show_bug.cgi?id=8155Signed-off-by: NOlaf Kirch <olaf.kirch@oracle.com>
Acked-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -413,7 +413,7 @@ static int do_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;
 			switch (rthdr->type) {
 			case IPV6_SRCRT_TYPE_0: