gdbusauth: fix crash when server data send returns NULL
gdbusauth: fix crash when server data send returns NULL

_g_dbus_auth_mechanism_server_data_send may fail in which case
we would endup getting a NULL data. In this case we should not
try to encode the data and simply let the state machine to continue.
The auth mechanism will change internally to REJECTED so we just
need to continue the iteration.

https://bugzilla.gnome.org/show_bug.cgi?id=775309

endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib