[php-src] Issue #20833: Division by zero in zif_mb_str_pad with UTF-32 and boolean pad string

Issue: https://github.com/php/php-src/issues/20833
Author: vi3tL0u1s

### Description

The following code:

```php
<?php
$utf8 = "test";
$utf32 = mb_convert_encoding($utf8, 'UTF-32', 'UTF-8');
var_dump(mb_str_pad($utf32, 44, true, STR_PAD_RIGHT, "UTF-32"));
```

Resulted in this output:
```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==2359430==ERROR: AddressSanitizer: FPE on unknown address 0x56162afce4dc (pc 0x56162afce4dc bp 0x7fffbdb7f4f0 sp 0x7fffbdb7f2c0 T0)
    #0 0x56162afce4dc in zif_mb_str_pad /path/to/php-src/ext/mbstring/mbstring.c:5880
    #1 0x56162bc8943b in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER /path/to/php-src/Zend/zend_vm_execute.h:1421
    #2 0x56162bdf34ee in execute_ex /path/to/php-src/Zend/zend_vm_execute.h:116441
    #3 0x56162be086cc in zend_execute /path/to/php-src/Zend/zend_vm_execute.h:121924
    #4 0x56162bf771fa in zend_execute_script /path/to/php-src/Zend/zend.c:1981
    #5 0x56162b992d50 in php_execute_script_ex /path/to/php-src/main/main.c:2645
    #6 0x56162b9931bb in php_execute_script /path/to/php-src/main/main.c:2685
    #7 0x56162bf7d17b in do_cli /path/to/php-src/sapi/cli/php_cli.c:951
    #8 0x56162bf7f8eb in main /path/to/php-src/sapi/cli/php_cli.c:1362
    #9 0x7f222035ed8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #10 0x7f222035ee3f in __libc_start_main_impl ../csu/libc-start.c:392
    #11 0x56162aa06eb4 in _start (/path/to/php-src/sapi/cli/php+0x606eb4)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: FPE /path/to/php-src/ext/mbstring/mbstring.c:5880 in zif_mb_str_pad
==2359430==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 01:08:08) (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