message: Fix lint error in sync client
diff: @@ -207,7 +207,7 @@ impl Client {
  const PING_INTERVAL: Duration = Duration::from_secs(2 * 60);
  const MAX_PINGS_OUT: u8 = 2;
  - let mut last = Instant::now() - MIN_FLUSH_BETWEEN;
  + let mut last = Instant::now().checked_sub(MIN_FLUSH_BETWEEN).unwrap();
  // Wait until at least one message is buffered.
  loop {
  