Update isValidDuration function

Signed-off-by: Yang Jiao <yang.jiao@broadcom.com>

src/server/v2.0/handler/robot.go

@@ -374,7 +374,7 @@ func isValidLevel(l string) bool {
 }
 
 func isValidDuration(d int64) bool {
-	return d >= int64(-1) && d != 0 && d < math.MaxInt32
+	return d == -1 || (d > 0 && d <= math.MaxInt32)
 }
 
 // validateName validates the robot name, especially '+' cannot be a valid character