trivial: Allow a NULL destdir in as_utils_find_icon_filename_full()
trivial: Allow a NULL destdir in as_utils_find_icon_filename_full()

hughsie/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib