[IPV6]: Fix kernel OOPs when setting sticky socket options.

Bug noticed by Remi Denis-Courmont <rdenis@simphalempin.com>.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/exthdrs.c

@@ -635,6 +635,7 @@ ipv6_renew_options(struct sock *sk, struct ipv6_txoptions *opt,
 	struct ipv6_txoptions *opt2;
 	int err;
 
+	if (opt) {
 		if (newtype != IPV6_HOPOPTS && opt->hopopt)
 			tot_len += CMSG_ALIGN(ipv6_optlen(opt->hopopt));
 		if (newtype != IPV6_RTHDRDSTOPTS && opt->dst0opt)
@@ -643,6 +644,8 @@ ipv6_renew_options(struct sock *sk, struct ipv6_txoptions *opt,
 			tot_len += CMSG_ALIGN(ipv6_optlen(opt->srcrt));
 		if (newtype != IPV6_DSTOPTS && opt->dst1opt)
 			tot_len += CMSG_ALIGN(ipv6_optlen(opt->dst1opt));
+	}
+
 	if (newopt && newoptlen)
 		tot_len += CMSG_ALIGN(newoptlen);
 
@@ -659,25 +662,25 @@ ipv6_renew_options(struct sock *sk, struct ipv6_txoptions *opt,
 	opt2->tot_len = tot_len;
 	p = (char *)(opt2 + 1);
 
-	err = ipv6_renew_option(opt->hopopt, newopt, newoptlen,
+	err = ipv6_renew_option(opt ? opt->hopopt : NULL, newopt, newoptlen,
 				newtype != IPV6_HOPOPTS,
 				&opt2->hopopt, &p);
 	if (err)
 		goto out;
 
-	err = ipv6_renew_option(opt->dst0opt, newopt, newoptlen,
+	err = ipv6_renew_option(opt ? opt->dst0opt : NULL, newopt, newoptlen,
 				newtype != IPV6_RTHDRDSTOPTS,
 				&opt2->dst0opt, &p);
 	if (err)
 		goto out;
 
-	err = ipv6_renew_option(opt->srcrt, newopt, newoptlen,
+	err = ipv6_renew_option(opt ? opt->srcrt : NULL, newopt, newoptlen,
 				newtype != IPV6_RTHDR,
-				(struct ipv6_opt_hdr **)opt2->srcrt, &p);
+				(struct ipv6_opt_hdr **)&opt2->srcrt, &p);
 	if (err)
 		goto out;
 
-	err = ipv6_renew_option(opt->dst1opt, newopt, newoptlen,
+	err = ipv6_renew_option(opt ? opt->dst1opt : NULL, newopt, newoptlen,
 				newtype != IPV6_DSTOPTS,
 				&opt2->dst1opt, &p);
 	if (err)