[fix] stack could mistakenly be seen as a FileDirectory if the contained...

... extension was ISO

The problem solved here is that CFileDirectoryFactory *just* looks at the extension, without taking into account the 'protocol'. So a stack of .ISOs is seen by CFileDirectoryFactory as a single ISO, which then assumes that we're trying to open an ISO Directory (returns CUDFDirectory). 

This 'bug' has been introduced in 850a7e00a7a597df450ba8d0c73bfdbc6aace4af - the changes for ISO/UDF "Browse Into". 

In order to make the fix simple, it's better to ignore the creation of a FileDirectory when we're dealing with a stack . Anyway, when playing back a stack part, it will be treated as a separate file, and at that time lead to the (correct) opening of a UDFDirectory for that particular individual ISO.