[php-src] Issue #21252: Segfault in zend_signal_handler_unblock when using APCu

Issue: https://github.com/php/php-src/issues/21252
Author: Appla

### Description

When running PHP with APCu enabled, the process occasionally segfaults inside `zend_signal_handler_unblock`. The crash seems related to signal queue handling (`SIGG(phead)` unexpectedly `NULL`) during APCu cache operations.

This happens in our production environment under real traffic. We cannot provide a minimal reproducer, but we do have a core dump showing the fault.

Backtrace (from core dump)
```
[0] zend_signal_handler_unblock+46 at Zend/zend_signal.c:163
[1] apc_cache_runlock+32 at apc_cache.h:298
[2] apc_cache_runlock+49 at apc_cache.h:296
[3] apc_cache_exists+248 at apc_cache.c:890
[4] zif_apcu_exists+331 at php_apc.c:680
***
```

### PHP Version

```plain
PHP 8.3.28 (cli) (built: Dec 18 2025 15:29:12) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.28, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.28, Copyright (c), by Zend Technologies
```

### Operating System

Rocky Linux 8