Allow NULL to be passed to destructors

In this case, the destructor will be a no-op. This is similar to free(3)'s
behavior and allows to omit a NULL check for callers.

device.c

@@ -70,6 +70,10 @@ void liftoff_device_destroy(struct liftoff_device *device)
 {
 	struct liftoff_plane *plane, *tmp;
 
+	if (device == NULL) {
+		return;
+	}
+
 	close(device->drm_fd);
 	liftoff_list_for_each_safe(plane, tmp, &device->planes, link) {
 		plane_destroy(plane);

layer.c

@@ -19,6 +19,10 @@ struct liftoff_layer *liftoff_layer_create(struct liftoff_output *output)
 
 void liftoff_layer_destroy(struct liftoff_layer *layer)
 {
+	if (layer == NULL) {
+		return;
+	}
+
 	layer->output->layers_changed = true;
 	if (layer->plane != NULL) {
 		layer->plane->layer = NULL;

output.c

@@ -37,6 +37,10 @@ struct liftoff_output *liftoff_output_create(struct liftoff_device *device,
 
 void liftoff_output_destroy(struct liftoff_output *output)
 {
+	if (output == NULL) {
+		return;
+	}
+
 	liftoff_list_remove(&output->link);
 	free(output);
 }