Segfault in mb_strrpos / mb_strripos when using negative offset and ASCII encoding

### Description

The following code:

```php
<?php
var_dump(mb_strrpos('foo', 'foo', -1, 'ASCII'));
var_dump(mb_strripos('foo', 'foo', -1, 'ASCII'));
```

Resulted in this output:
```
Segmentation fault
```

But I expected this output instead:
```
bool(false)
bool(false)
```

https://3v4l.org/BlLRn

### PHP Version

PHP 8.2.0

### Operating System

_No response_