Fixed a MediaPlayer.Play() bug on windows

re: this issue: https://github.com/mono/MonoGame/issues/2330
where only the first call to MediaPlayer.Play() takes effect, and subsequent calls will simply re-play the first song.

I've found that if a session is closed after being stopped, before the next song's topology is loaded, the correct song will be loaded and played, and the SharpDX exception is no longer thrown.
I have tested locally (windows 7) and it appears to work.