runtime: skip negative sleep durations in sleepTicks

sleepTicks calls machineLightSleep with the duration cast to an unsigned
integer. This underflow for negative durations.

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

src/runtime/runtime_rp2040.go

@@ -31,7 +31,7 @@ func nanosecondsToTicks(ns int64) timeUnit {
 }
 
 func sleepTicks(d timeUnit) {
-	if d == 0 {
+	if d <= 0 {
 		return
 	}