Fix heap-use-after-free with duplicate xkb layouts

Debian's gnome-control-center can crash when show-all-sources is
enabled.  When parse_end_element in gnome-xkb-info.c encounters
duplicate layouts, it will free the memory for the first layout while it
is still in a hash table.

https://bugzilla.gnome.org/show_bug.cgi?id=785320