[ticket/11480] PM : "Unknown folder" returned when inbox folder is full

https://tracker.phpbb.com/browse/PHPBB3-11480

PHPBB3-11480
From your description it sounds to me, that the correct fix would be to extend the check in the get_folder function from:
```php
	if ($folder_id !== false && !isset($folder[$folder_id]))
	{
		trigger_error('UNKNOWN_FOLDER');
	}
```
to something like:
```php
	if ($folder_id !== false && $folder_id !== PRIVMSGS_HOLD_BOX && !isset($folder[$folder_id]))
	{
		trigger_error('UNKNOWN_FOLDER');
	}
```

I tested this and in the end correctly `NO_AUTH_READ_HOLD_MESSAGE` is displayed