data-device: Only send out NULL selection if we have a resource

If the current keyboard focus client doesn't have a resource for the
data_device, don't send out the selection event.

src/data-device.c

@@ -356,6 +356,7 @@ destroy_selection_data_source(struct wl_listener *listener, void *data)
 	if (focus) {
 		data_device = find_resource(&seat->drag_resource_list,
 					    focus->client);
+		if (data_device)
 			wl_data_device_send_selection(data_device, NULL);
 	}
 }