Fix crashes and cleanup warnings

[heftig]

I'm familiar with C but not with C++. I massaged the code (with some help from valgrind and asan) until it no longer crashed compressing the soundfonts I had, which are:

• KOR Soundfont - GM Complete 4.51.sf2
• OmegaGMGS2.sf2

I also cleaned up some warnings which appeared with -Wall. You might want to cherry-pick one or more of the individual commits even if you reject the PR as a whole.