fastfat: explorer crashes when clicking drives in My Computer

Created an attachment (id=6800)
debug log

This bug was regress-tested on Vbox 4.1.4 to r51409. To replicate:
1. Make sure your ROS vm has floppy controller added;
2. Attach the floppy image i linked;
3. Boot to third stage and open My Computer. Click on any drive, including floppy or cdrom.

You can also replicate this crash via New Device wizard, via pointing driver manually and clicking on My Computer within the tree.