Calling zsm_stopmusic() no music playing causes issues

[mwiedmann]

If you call zsm_stopmusic() before any music is playing, it nearly crashes your program. It seems to run over some memory probably because some things are not initialized (like zsm_chanmask). Perhaps is can detect no music is playing and just return.

[mwiedmann]

I'd actually like to amend this. I don't think zsm_stopmusic() works right at all. It does stop the music from playing put it spams a bunch of things into VRAM.