gdbus: Fix invalid memory access during interface removal

gdbus: Fix invalid memory access during interface removal

If an interface is removed from the root path during the same mainloop
iteration that it was added we need to check for data->added before
doing the check for data->parent == NULL in the remove_interface()
function. Otherwise the added interface doesn't get removed from the
data->added list and will result in accessing freed memory:

==337== Invalid read of size 8
==337==    at 0x4F65AFA: dbus_message_iter_append_basic (in /usr/lib64/libdbus-1.so.3.7.1)
==337==    by 0x1247B5: append_interface (object.c:556)
==337==    by 0x4C8DC5C: g_slist_foreach (gslist.c:840)
==337==    by 0x1261F7: process_changes (object.c:594)
==337==    by 0x126372: generic_unregister (object.c:997)
==337==    by 0x4F69669: ??? (in /usr/lib64/libdbus-1.so.3.7.1)
==337==    by 0x4F5CE51: dbus_connection_unregister_object_path (in /usr/lib64/libdbus-1.so.3.7.1)
==337==    by 0x125E81: object_path_unref (object.c:1236)
==337==    by 0x126136: g_dbus_unregister_interface (object.c:1361)
==337==    by 0x14CDF0: service_exit (service.c:581)
==337==    by 0x177556: plugin_cleanup (plugin.c:242)
==337==    by 0x12221F: main (main.c:559)
==337==  Address 0x5bc1550 is 0 bytes inside a block of size 56 free'd
==337==    at 0x4A079AE: free (vg_replace_malloc.c:427)
==337==    by 0x4C7850E: g_free (gmem.c:252)
==337==    by 0x125DB0: remove_interface (object.c:671)
==337==    by 0x125E3B: object_path_unref (object.c:1230)
==337==    by 0x126136: g_dbus_unregister_interface (object.c:1361)
==337==    by 0x14CDF0: service_exit (service.c:581)
==337==    by 0x177556: plugin_cleanup (plugin.c:242)
==337==    by 0x12221F: main (main.c:559)