compositor-wayland: Use the correct list pointer when freeing shm buffers

The wl_list_for_each operation on the free_buffers list should use
free_link not link, which is a different list.

This fixes a crash when entering fullscreen mode when using the pixman
renderer on the wayland back-end.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>

src/compositor-wayland.c

@@ -740,7 +740,7 @@ wayland_output_resize_surface(struct wayland_output *output)
 	}
 
 	/* Throw away any remaining SHM buffers */
-	wl_list_for_each_safe(buffer, next, &output->shm.free_buffers, link)
+	wl_list_for_each_safe(buffer, next, &output->shm.free_buffers, free_link)
 		wayland_shm_buffer_destroy(buffer);
 	/* These will get thrown away when they get released */
 	wl_list_for_each(buffer, &output->shm.buffers, link)