runtime: handle negative sleep times

This change fixes the edge case where a negative sleep time is provided.
When this happens, the call now returns immediately (as specified by the docs for time.Sleep).

testdata/stdlib.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"strings"
 	"syscall"
+	"time"
 )
 
 func main() {
@@ -29,6 +30,10 @@ func main() {
 	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
 	fmt.Println("strings.Replace:", strings.Replace("An example string", " ", "-", -1))
 
+	// package time
+	time.Sleep(time.Millisecond)
+	time.Sleep(-1) // negative sleep should return immediately
+
 	// Exit the program normally.
 	os.Exit(0)
 }