gliststore: Store validity of last_position explicitly
gliststore: Store validity of last_position explicitly

Rather than storing it as an invalid value in last_position, store it as
a separate boolean.

This introduces no functional changes, but should fix some warnings from
MSVC.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1500

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib