func (rcv *UDP) Addr() net.Addr {
	if rcv.conn == nil {
		return nil
	}
	return rcv.conn.LocalAddr()
}
func (rcv *UDP) Addr() net.Addr {
	if rcv.conn == nil {
		return nil
	}
	return rcv.conn.LocalAddr()
}
// Addr returns binded socket address. For bind port 0 in tests