index += 1;
    if (index < strlen) {
      ch = str.charAt(index);
    } else {
      ch = 0;
    }

If I was using kotlin, I'd do it even cleaner:

    inline fun String.at(index: Int): Char? =
      if (index >= 0 && index < length) charAt(index)
      else null

    [...]

    index += 1
    ch = str.at(index) ?: 0;

The bytecode is exactly the same, but I’d argue it’s much cleaner