The script xdg-icon-resource tries to create a GTK icon cache when installing
icons. The problem here is that there might not have been an icon cache there in
the first place.

Suppose then that another package installs an icon, but does not update the icon
cache (this happens frequently in Debian). Then GTK will not see the new icon
because it uses the outdated cache. Thus, xdg-icon-resource breaks unrelated
packages.

My proposal for fixing this is to check if a cache exists before trying to
update it.