[php-src] Issue #20855: Assertion failure in zend_hash_del with HashTable refcount inconsistency

Issue: https://github.com/php/php-src/issues/20855
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
for ($i =3; $i <= 6567; $i++) {
    class A {
        function __destruct() {}
    }
    $c = new ReflectionClass('A');
    ob_start(function () use (&$c) {
        $arr = array(new stdClass);
        $arr[0]->b != serialize(serialize($GLOBALS));
    }, 1);
    $c = $array = array($c);
    unset($array[$f]);
}
```

```
Warning: Undefined variable $f in /path/to/poc.php on line 12

Warning: Undefined property: stdClass::$b in /path/to/poc.php on line 9
php: /path/to/php-src/Zend/zend_hash.c:1556: zend_hash_del: Assertion `(zend_gc_refcount(&(ht)->gc) == 1) || ((ht)->u.flags & (1<<6))' failed.
Aborted
```

### Commit

```
5472cac806a
```

### Configurations

```bash
CC="clang" CXX="clang++" CFLAGS="-fsanitize=address -g -O0" CXXFLAGS="-fsanitize=address -g -O0" ./configure --enable-debug --enable-address-sanitizer --disable-shared --with-pic
```


### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Jan  7 2026 13:33:43) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
    with Zend OPcache v8.6.0-dev, Copyright (c), by Zend Technologies
```

### Operating System

Ubuntu 22.04