Fixes for ISO9660 directory hierarchy creation and non-valid ISO file check

I tryed to create a ISO9660 directory hierarchy from a bottom-up approach, but that was not possible. I saw that some internal info (root and level) only propagates to children nodes when I use the top-down way, so I made a fix for that.

I was looking for a way to check if a file is a valid ISO file.
When I try to open a text file as an ISO file using FileSystemManager.resolveFile (see the test), a null value is inserted in the queue of EntryInterator and a NPE is raised.
The solution works fine.