Fix EZP-22958: invalid values in subtree criterion handler

JIRA: https://jira.ez.no/browse/EZP-22958

Add proper validation of subtree format (`/1/2/3/`) for all elements in the array $value passed to the Subtree criterion.

Note that this does force the pathString to end with a forward-slash (`/`).
This could be written in a simpler way, like:

```php
if ( preg_match( '/^(\/\w+)+\/$/', $pathString ) !== 1 )
{
    ...
}
```