thunar: Fix segfault in case no icon-theme is selected/installed

The patch was sent to xfce bug list [1]. As long as it not applied, make
thunar work for us.

[1] https://bugzilla.xfce.org/show_bug.cgi?id=7880



Signed-off-by: Andreas Mueller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>