wlr_output_power_management_v1: Init output_power->link

This makes sure the `wl_list_remove(&output_power->link)` in
`output_power_destroy()` does not crash even when the output_power never
got added to a list. This can e.g. happen in the `mgmt->output ==
output` error path of `output_power_manager_get_output_power`.

types/wlr_output_power_management_v1.c

@@ -123,6 +123,7 @@ static void output_power_manager_get_output_power(struct wl_client *client,
 	}
 	output_power->output = output;
 	output_power->manager = manager;
+	wl_list_init(&output_power->link);
 
 	uint32_t version = wl_resource_get_version(manager_resource);
 	output_power->resource = wl_resource_create(client,