player: request new song only if there is no pending command

Request the next song from the playlist (by clearing pc.next_song)
only if the player command is empty.  If it is not, the player may be
clearing the song that has already been queued, leading to an
assertion failure.

src/player_thread.c

@@ -314,7 +314,8 @@ static void do_play(void)
 		}
 
 		if (decoder_is_idle() && !player.queued &&
-		    pc.next_song != NULL) {
+		    pc.next_song != NULL &&
+		    pc.command == PLAYER_COMMAND_NONE) {
 			/* the decoder has finished the current song;
 			   request the next song from the playlist */
 			pc.next_song = NULL;