Description:
------------
mb_scrub() silently truncates anything after a null byte in a string.

This bug has existed since the function was created.

The arguments are read correctly, but the returned value ignores the length:
https://github.com/php/php-src/blob/30668755b64aa732246d952451f89d1fcfe581f0/ext/mbstring/mbstring.c#L4990

Test script:
---------------
<?php
$str = "before\0after";
var_dump(mb_scrub($str, 'latin1'));
var_dump(mb_scrub($str, 'utf-8'));
var_dump(mb_scrub($str, 'ascii'));

Expected result:
----------------
There should be a null byte in each of these.

string(12) "beforeafter"
string(12) "beforeafter"
string(12) "beforeafter"

Actual result:
--------------
string(6) "before"
string(6) "before"
string(6) "before"