xrick: Fix a race condition that could result in audio stopping
get_more() can disable audio when there's nothing more to genreate
but that can get called (and "fail") _before_ we set the flag
that audio is turned on. If this occurs, we will incorrecly
set the "audio on" flag after the failure which will prevent
the mixer from being re-started.
(We're at the mercy of thread scheduling)
Change-Id: I4f6c50a71c7ad685ff45f775e7e4b1c61b8b7777