[php-src] Issue #21927: Use-after-free of self-freeing MultipleIterator children

Issue: https://github.com/php/php-src/issues/21927
Author: iluuu1994

### Description

Originally reported by @flex0geek.

The following code:

```php
<?php

class FreeSelfOnRewind implements Iterator {
    public function __construct(private MultipleIterator $parent) {}

    public function rewind(): void {
        $this->parent->detachIterator($this);
        var_dump($this);
    }

    public function current(): mixed { return 0; }
    public function key(): mixed { return 0; }
    public function next(): void {}
    public function valid(): bool { return false; }
}

$mi = new MultipleIterator();
$mi->attachIterator(new FreeSelfOnRewind($mi));
$mi->rewind();
```

Resulted in this output:
```
Use-after-free
```

The same goes for various other `MultipleIterator` methods that call methods on the child iterator without addref.

### PHP Version

```plain
8.4+
```

### Operating System

_No response_