prefs: Do not leave junk values in unused button_layout fields

MetaButtonLayout is extremely unfriendly for introspection: its fields
are arrays of a fixed length, but the actual length is determined by
a custom stop value (e.g. not NULL / 0).
Without API changes this will never work nicely in introspection, but
we can at least make it work; start by filling up unused fields with
the stop value rather than leaving it at random values.

https://bugzilla.gnome.org/show_bug.cgi?id=689263