Do not play another track onMusicOver if stopped.

Due to a change in SDL_mixer, onMusicOver is now called when
the music is stopped with Mix_HaltMusic. When the music is stopped
we do not want onMusicOver starting another background track.

Fixes GH-129.
Merge pull request #247 from TheCycoONE/sdl2_haltmusic

Do not play another track onMusicOver if stopped.