Do not restart music that is already playing

[Loobinex]

Fixes #4505

[cerwym]

Can you change this so play_music is the only function modified and the callers remain dumb;

std::lock_guard<std::mutex> guard(g_mix_mutex);
    if (strcmp(game.music_fname, fname) == 0) {
JUSTLOG("doing nothing because %d is still playing", game.music_track);
        return true;
    }
    game.music_track = -1;

Would give you the same effect