zend_memnstr: Assertion `end >= p' failed.

### 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