housekeeping: Optimise for "do nothing" when cleaning thumbnails

If max_age and max_size are both -1, thumbnail cleaning
is disabled, so there is no need to scan the cache
directories.

https://bugzilla.gnome.org/show_bug.cgi?id=625609
https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/505085