runtime: don't call sleepTicks with a negative duration

There are rare cases where this can happen, see for example
#4568

builder/sizes_test.go

@@ -41,9 +41,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4568, 280, 0, 2268},
-		{"microbit", "examples/serial", 2868, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 6104, 1484, 116, 6832},
+		{"hifive1b", "examples/echo", 4580, 280, 0, 2268},
+		{"microbit", "examples/serial", 2888, 388, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 6124, 1484, 116, 6832},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the

src/runtime/scheduler.go

@@ -230,13 +230,15 @@ func scheduler(returnAtDeadlock bool) {
 					println("---   timer waiting:", tim, tim.whenTicks())
 				}
 			}
-			sleepTicks(timeLeft)
-			if asyncScheduler {
-				// The sleepTicks function above only sets a timeout at which
-				// point the scheduler will be called again. It does not really
-				// sleep. So instead of sleeping, we return and expect to be
-				// called again.
-				break
+			if timeLeft > 0 {
+				sleepTicks(timeLeft)
+				if asyncScheduler {
+					// The sleepTicks function above only sets a timeout at
+					// which point the scheduler will be called again. It does
+					// not really sleep. So instead of sleeping, we return and
+					// expect to be called again.
+					break
+				}
 			}
 			continue
 		}