Race condition when closing the Jack plugin

alsa-project/alsa-plugins issue #11 was opened from jeberger:

There is a potential race condition that can cause segmentation faults if the Jack process callback is called between the call to `snd_pcm_jack_stop` and `snd_pcm_jack_hw_free`. This was introduced by revision badd921.

Issue URL : https://github.com/alsa-project/alsa-plugins/issues/11 Repository URL: https://github.com/alsa-project/alsa-plugins