FAT: Fixed some dirent pointer access issues.

* Fixed #9806 which Ingo had analyzed perfectly, but then forgot to
  fix it.
* Fixed CID 603015.
FAT: improved last fix.

* Don't use filename at all, since it's not clear if puni has already
  been set at this point.
* Also, fixed warning *vnid is ino_t, not a pointer.
FAT: minor style cleanup.