Use duration as default when subtracting duration in accurate timer.

If the duration subtraction in the accurate timer results in an
overflow, just use the original duration instead of expecting an error.

src/midi/midir.rs

@@ -317,9 +317,11 @@ impl<T: Timer> Timer for AccurateTimer<T> {
         match self.last_instant {
             Some(last_instant) => {
                 self.last_instant = Some(last_instant.add(duration));
+
+                // Subtract the duration unless it would be an overflow. If so, use the original duration.
                 duration = duration
                     .checked_sub(Instant::now().duration_since(last_instant))
-                    .expect("duration should not be less during subtraction")
+                    .unwrap_or(duration);
             }
             None => self.last_instant = Some(time::Instant::now()),
         };