Fix: cancel a notif. iter. finalized during its "next" method

Fix: cancel a notif. iter. finalized during its "next" method

It is possible that the user's "next" method, somehow, cancels its own
notification iterator. This can happen, for example, when the user's
method removes the port on which there's the connection from which the
notification iterator was created. In this case, said connection is
ended, and all its notification iterators are finalized.

In ensure_queue_has_notifications(), after calling the user's "next"
method, if there's no error, we check if the notification iterator is
finalized. If so, we return the BT_NOTIFICATION_ITERATOR_STATUS_CANCELED
status directly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>