gdbus: Fix invalid memory access while unregistering

gdbus: Fix invalid memory access while unregistering

If an interface is added and removed on the same mailoop iteration,
ObjectManager would try to send InterfacesAdded signal while running the
idler because the interface was added to data->added list.

This is easily reproduced by forcing an error path in a plugin
registration, like on sap_server_register(), resulting in the following
error:

==11795== Invalid read of size 4
==11795==    at 0x496F592: dbus_message_iter_append_basic (dbus-message.c:2598)
==11795==    by 0x117B39: append_interface (object.c:554)
==11795==    by 0x48955E7: g_slist_foreach (gslist.c:840)
==11795==    by 0x11923B: process_changes (object.c:592)
==11795==    by 0x11956D: generic_unregister (object.c:980)
==11795==    by 0x4973BAC: _dbus_object_tree_unregister_and_unlock (dbus-object-tree.c:516)
==11795==    by 0x4965240: dbus_connection_unregister_object_path (dbus-connection.c:5776)
==11795==    by 0x1178A5: object_path_unref (object.c:1219)
==11795==    by 0x118517: g_dbus_unregister_interface (object.c:1344)
==11795==    by 0x19AF5B: sap_exit (sap.c:385)
==11795==    by 0x13E9E2: sap_server_register (server.c:1428)
==11795==    by 0x13C092: sap_server_probe (manager.c:44)

With this patch we don't send the InterfacesAdded signal, removing it
from data->added while unregistering.