[php-src] Issue #20836: Stack overflow in mbstring variable conversion with recursive array references

Issue: https://github.com/php/php-src/issues/20836
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
$a[] = $b[] = &$a;
var_dump(mb_convert_variables('utf-8', 'utf-8', $a), $a);
```

Resulted in this output:
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==2593609==ERROR: AddressSanitizer: stack-overflow on address 0x7ffdba5e0ff8 (pc 0x5643a833ccc2 bp 0x7ffdba5e1010 sp 0x7ffdba5e0fe0 T0)
    #0 0x5643a833ccc2 in zend_mm_get_next_free_slot /path/to/php-src/Zend/zend_alloc.c:1320
    #1 0x5643a833d5f9 in zend_mm_alloc_small /path/to/php-src/Zend/zend_alloc.c:1409
    #2 0x5643a833db62 in zend_mm_alloc_heap /path/to/php-src/Zend/zend_alloc.c:1487
    #3 0x5643a8344adf in _emalloc /path/to/php-src/Zend/zend_alloc.c:2783
    #4 0x5643a86431bd in zend_array_dup /path/to/php-src/Zend/zend_hash.c:2458
    #5 0x5643a77b94a2 in mb_recursive_convert_variable /path/to/php-src/ext/mbstring/mbstring.c:3787
    #6 0x5643a77b99fe in mb_recursive_convert_variable /path/to/php-src/ext/mbstring/mbstring.c:3814
    #7 0x5643a77b99fe in mb_recursive_convert_variable /path/to/php-src/ext/mbstring/mbstring.c:3814
    ... (hundreds of recursive calls)
    #248 0x5643a77b99fe in mb_recursive_convert_variable /path/to/php-src/ext/mbstring/mbstring.c:3814

SUMMARY: AddressSanitizer: stack-overflow /path/to/php-src/Zend/zend_alloc.c:1320 in zend_mm_get_next_free_slot
==2593609==ABORTING
```

### Commit

```
cb63e4f998b
```

### 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  5 2026 10:22:05) (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