[php-src] Issue #20832: mbstring: assertion failure in mb_wchar_to_sjismac with MacJapanese encoding

Issue: https://github.com/php/php-src/issues/20832
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
class Test
{
    public function __get($var)
    { 
      if (true) { 
        $var= $ar; 
        $this->{$var}!=print bin2hex(mb_substr("abc\n85xx\x85\xAC\x85\xAD", -2, $test->t1, 'MacJapanese')); 
      }
    }
}
$test = new Test;
$test->teht;
```

Resulted in this output:
```
php: /path/to/php-src/ext/mbstring/libmbfl/filters/mbfilter_cjk.c:5675: mb_wchar_to_sjismac: Assertion `out <= limit' failed.
Aborted
```

### Commit

```
cb63e4f998b
```

### Configurations

```bash
CC="clang" CXX="clang++" CFLAGS="-fsanitize=address -g -O0" CXXFLAGS="-fsanitize=address -g -O0" ./configure --enable-debug --enable-address-sanitizer --disable-shared --with-pic
```


### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Jan  5 2026 01:08:08) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies
```

### Operating System

Ubuntu 22.04