output/alsa: drain the whole `ring_buffer`, not just one period

This fixes a problem which caused a failure with snd_pcm_writei()
because snd_pcm_drain() had already been called in the previous
iteration.  This commit makes sure that snd_pcm_drain() is only called
after the final snd_pcm_writei() call.

This fixes discarded samples at the end of playback.

NEWS

@@ -2,6 +2,7 @@ ver 0.21.3 (not yet released)
 * output
   - alsa: fix crash bug
   - alsa: fix stuttering at start of playback
+  - alsa: fix discarded samples at end of song
   - alsa: clear error after reopening device
 * log: default to journal if MPD was started as systemd service
 

src/output/plugins/AlsaOutputPlugin.cxx

@@ -749,11 +749,10 @@ AlsaOutput::DrainInternal()
 						 snd_strerror(-frames_written));
 		}
 
-		if (!period_buffer.IsEmpty())
-			/* need to call WriteFromPeriodBuffer() again
-			   in the next iteration, so don't finish the
-			   drain just yet */
-			return false;
+		/* need to call CopyRingToPeriodBuffer() and
+		   WriteFromPeriodBuffer() again in the next
+		   iteration, so don't finish the drain just yet */
+		return period_buffer.IsEmpty();
 	}
 
 	/* .. and finally drain the ALSA hardware buffer */