>> ненастоящие Сишники
> Что-то всё чаще и чаще мелькает бубнеж этой мантры. Не надоело?

Смотрите комментарий над проблемной функцией:

/*-
 * Pseudocode:
 *
 * function ossl_punycode_decode
 *  let n = initial_n
 *  let i = 0
 *  let bias = initial_bias
 *  let output = an empty string indexed from 0
 *  consume all code points before the last delimiter (if there is one)
 *    and copy them to output, fail on any non-basic code point
 *  if more than zero code points were consumed then consume one more
 *    (which will be the last delimiter)
 *  while the input is not exhausted do begin
 *    let oldi = i
 *    let w = 1
 *    for k = base to infinity in steps of base do begin
 *      consume a code point, or fail if there was none to consume
 *      let digit = the code point's digit-value, fail if it has none
 *      let i = i + digit * w, fail on overflow
 *      let t = tmin if k <= bias {+ tmin}, or
 *              tmax if k >= bias + tmax, or k - bias otherwise
 *      if digit < t then break
 *      let w = w * (base - t), fail on overflow
 *    end
 *    let bias = adapt(i - oldi, length(output) + 1, test oldi is 0?)
 *    let n = n + i div (length(output) + 1), fail on overflow
 *    let i = i mod (length(output) + 1)
 *    {if n is a basic code point then fail}
 *    insert n into output at position i
 *    increment i
 *  end
 */