Stop doesn't work if the audioDescription preferred sampleRate is different than detected

[cerupcat]

I'm setting up a audioDescription with 44100, but connecting to a bluetooth speaker with a sampleRate of 8800. When I try stopping the audio engine, audioUnitStreamFormatChanged gets called (not sure why) which then calls updateSampleRate causing the session to be rebuilt due to a change in sampleRate and audio starts again.

Adding if ( !_started) return; to the top of updateSampleRate fixes this, but I'm not sure it's the ideal location for the check as I'm not sure why updateSampleRate is being called on stop to begin with.