object: Don't invalidate closure if already invalid

In object_instance_finalize() we get two kinds of signal connections:
ones that are still connected, which we need to invalidate; and ones that
have just been disconnected and invalidated but whose invalidate idle
function has not yet run.

The second kind we just need to disconnect the idle handler and free the
ConnectData. We should not try to remove the invalidate notifier, since
it has already run. The first kind, we must remove the invalidate
notifier so that no idle invalidation is scheduled, and then invalidate
them, and also free the ConnectData.

Previously we were mixing the two and removing notifiers from both kinds.
This led to warnings about removing uninstalled invalidation notifiers.

https://bugzilla.gnome.org/show_bug.cgi?id=783951