bluetooth: Fix unregistering of the message handler

In cases, where codec initialisation may fail, we still try
unregistering the message handler. Do not try unregistering
the handler if it was not registered in the first place.

Part-of: <pulseaudio/pulseaudio!483>
3 jobs for fix-msg-handler-registration in 5 minutes and 28 seconds (queued for 2 seconds)