java datamodel: fixes getRootDirectory() when root has . and ..

.... and .. entries.

Fixes #175

This hasn't showed up in regression testing and Autopsy because:
- that method is not used in datamodel regression testing and directory tree
- in most cases (but not always, as in user's image) . and .. entries are not present for the root folder, at least in NTFS.  This could be a bug in addImg() or perhaps it is file system version dependent.