[php-src] Issue #23106: zend_memnstr: Assertion `end >= p' failed.

Issue: https://github.com/php/php-src/issues/23106
Author: lmaltsis

### Description

The following code:

```php
<?
mb_strpos("AA\xf0\x90", "xyz", 3);
```

Resulted in this output:
```
php: src/Zend/zend_operators.h:197: zend_memnstr: Assertion `end >= p' failed.
Aborted (core dumped)
```


### PHP Version

```plain
PHP 8.5.9 (cli) (built: Aug  7 2026 18:15:17) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.5.9, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.9, Copyright (c), by Zend Technologies

also triggers on trunk
```

### Operating System

Ubuntu 24.04