ipv6: Don't pass invalid dst_entry pointer to dst_release().

Make sure dst_release() is not called with error pointer. This is
similar to commit 4910ac6c ("ipv4:
Don't ip_rt_put() an error pointer in RAW sockets.").
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -503,6 +503,7 @@ static int tcp_v6_send_synack(struct sock *sk, struct request_sock *req,
 	dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);
 	if (IS_ERR(dst)) {
 		err = PTR_ERR(dst);
+		dst = NULL;
 		goto done;
 	}
 	skb = tcp_make_synack(sk, dst, req, rvp);