output/alsa: always redo DrainInternal() after writing

Draining isn't finished just because the period_buffer has run empty.
It is only finished after snd_pcm_drain() has succeeded.

src/output/plugins/AlsaOutputPlugin.cxx

@@ -766,7 +766,7 @@ AlsaOutput::DrainInternal()
 		/* need to call CopyRingToPeriodBuffer() and
 		   WriteFromPeriodBuffer() again in the next
 		   iteration, so don't finish the drain just yet */
-		return period_buffer.IsEmpty();
+		return false;
 	}
 
 	if (!written)