rtt: initialize min_rtt to zero

Follow-up to 3b758cb which changed the
initial min_rtt value from Duration::ZERO to Duration::MAX.

This could potentially cause an overflow, for example if there is no
previous RTT sample, and the CUBIC congestion avoidance logic is
triggered.

Switch back the initial value to ZERO.

quiche/src/recovery/rtt.rs

@@ -62,7 +62,7 @@ impl RttStats {
     pub(crate) fn new(max_ack_delay: Duration) -> Self {
         RttStats {
             latest_rtt: Duration::ZERO,
-            min_rtt: Minmax::new(Duration::MAX),
+            min_rtt: Minmax::new(Duration::ZERO),
             smoothed_rtt: INITIAL_RTT,
             rttvar: INITIAL_RTT / 2,
             first_rtt_sample: None,
@@ -114,6 +114,6 @@ impl RttStats {
     }
 
     pub(crate) fn min_rtt(&self) -> Option<Duration> {
-        self.min_rtt.ne(&Duration::MAX).then_some(*self.min_rtt)
+        self.min_rtt.ne(&Duration::ZERO).then_some(*self.min_rtt)
     }
 }