output_all: reset elapsed_time at song border

Another quirk fixed: after the last chunk of a song has been played,
the "elapsed_time" variable is set to the chunk's time stamp.  When
the client receives the PLAYER idle event and asks MPD for the current
time stamp, MPD will return the last time stamp of the previous song
when it hasn't played the first chunk of the current song yet.

src/player_thread.c

@@ -736,6 +736,8 @@ player_song_border(struct player *player)
 	music_pipe_free(player->pipe);
 	player->pipe = player->dc->pipe;
 
+	audio_output_all_song_border();
+
 	if (!player_wait_for_decoder(player))
 		return false;