[php-src] Issue #21738: UB in php_url_decode_ex

Issue: https://github.com/php/php-src/issues/21738
Author: xfourj

### Description

Consider the following snippet within ```php_url_decode_ex``` at ```ext/standard/url.c``` :

```php
else if (*data == '%' && src_len >= 2 && isxdigit((int) *(data + 1))
				 && isxdigit((int) *(data + 2))) {
```
In environments where char is signed by default—as is a common convention— high-bit bytes sign-extend to negative integers when being passed to ```isxdigit```. That violates the ISO C standard's requirement for ```isxdigit``` in ```<ctype.h>``` to be represenetable as an unsigned char (0 to 255), which leads to UB.

Example (taking ```\x80``` as the byte):

```php
<?php
urldecode("%\x80");
?>
```

Resulted in this output:
```
(gdb) p *(data+1)
$1 = -128 '\200'
```
Note that for libraries that implement ```isxdigit``` via maps, if no proper padding is done then OOB read might occur.  


### PHP Version

```plain
PHP 8.4+
```

### Operating System

_No response_