[php-src] Issue #9535: The behavior of mb_strcut in mbstring has been changed in PHP8.1

Issue: https://github.com/php/php-src/issues/9535 Author: pakutoma ### Description When using the mb_strcut function to cut out ISO-2022-JP encoded strings, depending on the number of bytes specified, the behavior is different in PHP 8.1 than before. The following code: ```php <?php $input = 'あaいb'; $bytes_length = 10; $encoding = "ISO-2022-JP"; $converted_str = mb_convert_encoding($input, $encoding, mb_internal_encoding()); $cut_str = mb_strcut($converted_str, 0, $bytes_length, $encoding); $reconverted_str = mb_convert_encoding($cut_str, mb_internal_encoding(), $encoding); var_dump($reconverted_str); ``` Resulted in this output: ``` string(5) "あa?" ``` But I expected this output instead: ``` string(4) "あa" ``` This behavior has changed since PHP 8.1. 3v4l: https://3v4l.org/FaVWR I believe the behavior up to PHP 8.0 is correct. This is because PHP 8.1 and later output contains a "?", which is not supposed to be there. ### PHP Version PHP 8.1.10 ### Operating System _No response_