player: reset pc.command when decoder startup fails

When the decoder failed to start, the function do_play() returned,
still having pc.command==PLAY.  This is because pc.command was reset
only when the decoder started up successfully.  Add another
player_command_finished() call in the error handler.

src/player_thread.c

@@ -246,6 +246,7 @@ static void do_play(void)
 	dc_start(&pc.notify, pc.next_song);
 	if (waitOnDecode(&player) < 0) {
 		quitDecode();
+		player_command_finished();
 		return;
 	}