zeroconf-publish: Fix crash when avahi_client_new() fails
When the module is loaded and avahi_client_new() fails because the client cannot connect, a shutdown of the module is scheduled. In parallel, the client_callback is called with AVAHI_ERR_DISCONNECTED and another connection attempt is made which also fails and triggers a second unload of the module. This crashes PA, because there is already an unload in progress. This patch fixes the problem by checking if an unload is already scheduled.