reactor: Account for timer weirdness

jitterbug/src/reactor.rs

@@ -218,10 +218,9 @@ impl ReactorState {
 
         let timeout = first_wakeup.map(|instant| {
             instant
-                .duration_since(now)
-                .as_millis()
-                .try_into()
-                .unwrap_or(i32::MAX)
+                .checked_duration_since(now)
+                .map(|duration| duration.as_millis().try_into().unwrap_or(i32::MAX))
+                .unwrap_or(0)
         });
 
         for (key, readiness) in self.poller.poll(timeout)? {