window: Avoid setting an invalid surface for the pointer

If we failed to load the pointer image to the surface then do not create a
buffer from a NULL surface and do not attach that to the device.

clients/window.c

@@ -1176,6 +1176,10 @@ set_pointer_image(struct input *input, uint32_t time, int pointer)
 
 	input->current_pointer_image = pointer;
 	surface = display->pointer_surfaces[pointer];
+
+	if (!surface)
+		return;
+
 	buffer = display_get_buffer_for_surface(display, surface);
 	wl_input_device_attach(input->input_device, time, buffer,
 			       pointer_images[pointer].hotspot_x,