Only call onMusicOver if the music naturally ends.

We were playing the bg music over the cutscene music.

This stops us from calling onMusicOver when we call Mix_HaltMusic. This is a
regression after upgrading to the latest SDL_mixer, due to a change in
functionality. (https://bugzilla.libsdl.org/show_bug.cgi?id=1610)

Fixes GH-129.