audiod: Avoid starting the wrong decoder on server restarts.

audiod: Avoid starting the wrong decoder on server restarts.

If para_server is restarted and starts streaming using another audio format than
the one used before the restart, para_audiod might end up starting the decoder
associated with the old audio format.

Fix this by invalidating current_audio_format_num in close_stat_pipe() which is
called when the server goes away.