ipv6: ip6_make_skb() needs to clear cork.base.dst

In my last patch, I missed fact that cork.base.dst was not initialized
in ip6_make_skb() :

If ip6_setup_cork() returns an error, we might attempt a dst_release()
on some random pointer.

Fixes: 862c03ee1d ("ipv6: fix possible mem leaks in ipv6_make_skb()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1735,6 +1735,7 @@ struct sk_buff *ip6_make_skb(struct sock *sk,
 	cork.base.flags = 0;
 	cork.base.addr = 0;
 	cork.base.opt = NULL;
+	cork.base.dst = NULL;
 	v6_cork.opt = NULL;
 	err = ip6_setup_cork(sk, &cork, &v6_cork, ipc6, rt, fl6);
 	if (err) {