Whenever you pause gameplay, the `PlayState` pauses the music.
But if you start a song from the main menu, pause during the 3,2,1
countdown, and then unpause, the main menu music will incorrectly
start playing again due to `FlxG.sound.music` still referencing it
before the gameplay song starts playing.
Only restart the music if it was actually playing to begin with by
storing the play state in `PlayState.openSubState` when we pause.
This hopefully avoids any other cases where the pause state gets
pushed while the music is stopped.
Mike Welsh
e90c37ad75