player/Thread: drain outputs at end of song in "single" mode

Without this, the Pause() call would drop the ring buffers and would
skip a considerable portion of the end of the song.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/824

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.23 (not yet released)
+* player
+  - drain outputs at end of song in "single" mode
 
 ver 0.21.22 (2020/04/02)
 * database

src/player/Thread.cxx

@@ -964,6 +964,12 @@ Player::SongBorder() noexcept
 	if (border_pause) {
 		paused = true;
 		pc.listener.OnBorderPause();
+
+		/* drain all outputs to guarantee the current song is
+		   really being played to the end; without this, the
+		   Pause() call would drop all ring buffers */
+		pc.outputs.Drain();
+
 		pc.outputs.Pause();
 		idle_add(IDLE_PLAYER);
 	}