avoid substaction overflow

Author: pvdrz

src/shims/time.rs

@@ -111,7 +111,10 @@ impl Clock {
                     (
                         Instant::Virtual { nanoseconds },
                         Clock::Virtual { nanoseconds: current_ns },
-                    ) => Duration::from_nanos(nanoseconds - current_ns.load(Ordering::Relaxed)),
+                    ) =>
+                        Duration::from_nanos(
+                            nanoseconds.saturating_sub(current_ns.load(Ordering::Relaxed)),
+                        ),
                     _ => panic!(),
                 },
             Time::RealTime(time) =>