zend_memnstr: Assertion `end >= p' failed.

[lmaltsis]

Description

The following code:

<?
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

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