[php-src] Issue #20894: Heap-buffer-overflow in zval_ptr_dtor_nogc during Fiber/Generator execution in destructor

Issue: https://github.com/php/php-src/issues/20894
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
class a {
    function __destruct() {
        static $gen = (function() {
            $from = (function () {
                !$v =& min(0, $x);
                x:$n[] = new a;
                y;
                yield;
            })();
            try { yield from $from; } finally { $y>next(); }
        })();
        $fiber = new Fiber(function () use ($gen) {
            $gen->current();
            $gen->next();
        });
        $fiber->start();
    }
}
new a;
```

### Crash Output (with `USE_ZEND_ALLOC=0`)

```
==PID==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x511000002ce9
READ of size 1 at 0x511000002ce9 thread T0
    #0 in zval_ptr_dtor_nogc /path/to/php-src/Zend/zend_variables.h:35
    #1 in ZEND_HANDLE_EXCEPTION_SPEC_HANDLER /path/to/php-src/Zend/zend_vm_execute.h:3450
    #2 in execute_ex /path/to/php-src/Zend/zend_vm_execute.h:116576
    #3 in zend_generator_resume /path/to/php-src/Zend/zend_generators.c:844
    #4 in zend_generator_ensure_initialized /path/to/php-src/Zend/zend_generators.c:900
    #5 in zim_Generator_current /path/to/php-src/Zend/zend_generators.c:955
    ...

0x511000002ce9 is located 9 bytes to the right of 224-byte region
allocated by thread T0 here:
    #0 in __interceptor_malloc
    #1 in __zend_malloc /path/to/php-src/Zend/zend_alloc.c:3543
    #2 in _emalloc /path/to/php-src/Zend/zend_alloc.c:2780
    #3 in ZEND_GENERATOR_CREATE_SPEC_HANDLER /path/to/php-src/Zend/zend_vm_execute.h:2354
    ...

SUMMARY: AddressSanitizer: heap-buffer-overflow /path/to/php-src/Zend/zend_variables.h:35 in zval_ptr_dtor_nogc
```

## Commit

`c518a6ba8bf7a3eb6551424c7ca90dfa06b01b15`

## Configurations

```
./configure --enable-debug --enable-address-sanitizer --disable-shared --with-pic --enable-mbstring --with-zlib
```


### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Jan  9 2026 14:43:36) (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