message: socks5 UDP association should return fake UDP bind addr if tcp only
  fixes shadowsocks/shadowsocks-android#2665
diff: @@ -118,6 +118,8 @@ impl Socks {
  let udp_bind_addr = self.udp_bind_addr.as_ref().unwrap_or(client_config);
  let udp_bind_addr = Arc::new(udp_bind_addr.clone());
  Some(udp_bind_addr)
  + } else if let Some(ref ua) = self.udp_bind_addr {
  + Some(Arc::new(ua.clone()))
  } else {
  None
  };
  