manager: Assert that cloning was successful

Cloning an ECalComponent can fail, and we need to be able to
catch these failures.

Assert that cloning was successful before procceeding with
the move.

src/core/gcal-manager.c

@@ -1099,6 +1099,8 @@ gcal_manager_move_event_to_source (GcalManager *self,
   clone = e_cal_component_clone (ecomponent);
   comp = e_cal_component_get_icalcomponent (clone);
 
+  g_assert (clone != NULL);
+
   e_cal_client_create_object_sync (gcal_calendar_get_client (calendar),
                                    comp,
                                    E_CAL_OPERATION_FLAG_NONE,