Account for nil directory entries in FS code.

Modify Ext3, Ext4 and XFS filesystem code to filter out nil directory entries.

https://bugzilla.redhat.com/show_bug.cgi?id=1170320
https://bugzilla.redhat.com/show_bug.cgi?id=1170319
Merge pull request #1169 from roliveri/ext3_dir

Account for nil directory entries in FS code.