[php-src] Issue #22322: spl_autoload() fails to load class with leading \

Issue: https://github.com/php/php-src/issues/22322
Author: jorgsowa

### Description

The following code:

```php
<?php
spl_autoload("DualIterator");
var_dump(class_exists("DualIterator", false));

spl_autoload("\\RecursiveDualIterator");
var_dump(class_exists("RecursiveDualIterator", false));
```

Resulted in this output:
```
bool(true)
bool(false)
```

But I expected this output instead:
```
bool(true)
bool(true)
```


### PHP Version

```plain
master
```

### Operating System

_No response_