message: Avoid panicing when substracting Instants
diff: @@ -640,9 +640,10 @@ impl LossRecovery {
  ack_delay: Duration,
  ) {
  let confirmed = self.confirmed_time.map_or(false, |t| t < send_time);
  - let sample = now - send_time;
  + if let Some(sample) = now.checked_duration_since(send_time) {
  rtt.update(&mut self.qlog, sample, ack_delay, confirmed, now);
  }
  + }
  /// Returns (acked packets, lost packets)
  pub fn on_ack_received<R>(
  