ringbufferpool: do not return a frame if nothing is mixed

This avoid an infinite loop in pulselayer cause during a short
moment the ringbufferpool will contains two binding mixed together
with nothing being mixed. So AudioLayer::getToPlay will just
lock forever and the pulselayer will not be able to stop the
mainloop

Change-Id: Ic6e316308de6ec37f9c7525e1601e14593fd17d1