[php-src] Issue #21134: ASan negative-size-param in preg_match_all() with \C + UTF-8 multibyte input

Issue: https://github.com/php/php-src/issues/21134
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
$str = " lisrep columns=2  $d)', 'avar_d?:Þßà";
preg_match_all("/([',$str\"]?.*)\\C/iu", $str, $str_instead);
```

Resulted in this output:
```
=================================================================
==648951==ERROR: AddressSanitizer: negative-size-param: (size=-1)
    #0 in __asan_memcpy (sapi/cli/php)
    #1 in zend_string_init Zend/zend_string.h:191
    #2 in zend_string_init_fast Zend/zend_string.h:199
    #3 in populate_match_value_str ext/pcre/php_pcre.c:944
    #4 in populate_match_value ext/pcre/php_pcre.c:957
    #5 in php_pcre_match_impl ext/pcre/php_pcre.c:1306
    #6 in php_do_pcre_match ext/pcre/php_pcre.c:1125
    #7 in zif_preg_match_all ext/pcre/php_pcre.c:1500
    #8 in ZEND_DO_ICALL_SPEC_RETVAL_UNUSED_HANDLER Zend/zend_vm_execute.h:1318
    #9 in execute_ex Zend/zend_vm_execute.h:110029
    #10 in zend_execute Zend/zend_vm_execute.h:115447
    #11 in zend_execute_script Zend/zend.c:1980
    #12 in php_execute_script_ex main/main.c:2648
    #13 in php_execute_script main/main.c:2688
    #14 in do_cli sapi/cli/php_cli.c:949
    #15 in main sapi/cli/php_cli.c:1360

SUMMARY: AddressSanitizer: negative-size-param in __asan_memcpy
==648951==ABORTING
```

## Commit

```
06dac62747f0819ebc110fd6ab4a90a0229bd2b6
```

## Build Configuration

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


### PHP Version

```plain
PHP 8.6.0-dev (cli) (built: Feb  5 2026 18:41:44) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.6.0-dev, Copyright (c) Zend Technologies
```

### Operating System

Ubuntu 22.04