runtime: guard against unsigned underflow in sleep time calculations

Signed-off-by: Elias Naur <mail@eliasnaur.com>

src/runtime/scheduler.go

@@ -202,10 +202,17 @@ func scheduler() {
 
 			var timeLeft timeUnit
 			if sleepQueue != nil {
-				timeLeft = timeUnit(sleepQueue.Data) - (now - sleepQueueBaseTime)
+				dt := (now - sleepQueueBaseTime)
+				timeLeft = timeUnit(sleepQueue.Data) - dt
+				if dt > timeUnit(sleepQueue.Data) {
+					timeLeft = 0
+				}
 			}
 			if timerQueue != nil {
 				timeLeftForTimer := timerQueue.whenTicks() - now
+				if now > timerQueue.whenTicks() {
+					timeLeftForTimer = 0
+				}
 				if sleepQueue == nil || timeLeftForTimer < timeLeft {
 					timeLeft = timeLeftForTimer
 				}