RandomController index calculation can go out of range

[robingustafsson]

The index calculation at https://github.com/loadimpact/jmeter-to-k6/blob/master/src/element/RandomController.js#L82 can go 1 above the max index. It generates the following code for a random controller with 3 sub-elements, which can generate an index value of 3:

const index = Math.floor(Math.random() * (3 + 1));
switch (index) {
  case 0:
    ...
    break;
  case 1:
    ...
    break;
  case 2:
    ...
    break;
  default:
    throw new Error("Unexpected random index: " + index);
}