Error out when the given duration is negative (#155)

Validate processInfo period & log a fatal error in the case of negative duration.

main.go

@@ -136,6 +136,9 @@ func processes() {
 }
 
 func processInfo(pid int, period time.Duration) {
+	if period < 0 {
+		log.Fatalf("Cannot determine CPU usage for negative duration %v", period)
+	}
 	p, err := process.NewProcess(int32(pid))
 	if err != nil {
 		log.Fatalf("Cannot read process info: %v", err)
@@ -152,10 +155,8 @@ func processInfo(pid int, period time.Duration) {
 	if v, err := p.CPUPercent(); err == nil {
 		fmt.Printf("cpu usage:\t%.3f%%\n", v)
 	}
-	if period > 0 {
-		if v, err := cpuPercentWithinTime(p, period); err == nil {
-			fmt.Printf("cpu usage (%v):\t%.3f%%\n", period, v)
-		}
+	if v, err := cpuPercentWithinTime(p, period); err == nil {
+		fmt.Printf("cpu usage (%v):\t%.3f%%\n", period, v)
 	}
 	if v, err := p.Username(); err == nil {
 		fmt.Printf("username:\t%v\n", v)