Xfburn crashes while trying to add a folder containing a unix socket.

Reproducible: Always

Steps to reproduce:
1. Create new composition
2. Select a directory that contains some sockets (for example /tmp on my system running X.org)
3. Add this directory to composition.

Actual results:
Xfburn crashes saying 'Segmentation fault'.

Expected results:
Xfburn should not crash and skip the problematic file (IMO).

Applies to rev. 22801 from svn.

Here is the output:
[szymon@maestro ~]$ xfburn
** Message: Using Thunar-VFS 0.3.3
** Message: looking for device [1]

** (xfburn:28922): CRITICAL **: Failed to get contents of file [/proc/scsi/sg/devices]

** (xfburn:28922): CRITICAL **: Failed to open /proc/scsi/sg/devices
** Message: device [1] found : (null) (/dev/sr0)
** Message: device [1] capabilities : CD-R CD-RW DVD-R
** Message: looking for device [2]
** Message: Requested device index [2] is out of bounds. All devices have been read.

** (xfburn:28922): WARNING **: unable to open directory : Error opening directory '/tmp/.ICE-unix/3201': Not a directory

(xfburn:28922): Gtk-CRITICAL **: gtk_tree_store_get_value: assertion `VALID_ITER (iter, tree_store)' failed

(xfburn:28922): GLib-GObject-WARNING **: gtype.c:3337: type id `0' is invalid

(xfburn:28922): GLib-GObject-WARNING **: can't peek value table for type `<invalid>' which is not currently referenced
Segmentation fault
[szymon@maestro ~]$