Add an assert for stb_vorbis_decode_memory call

[jgraj]

Return value of stb_vorbis_decode_memory is less than 0 for errors and we later malloc with that value, so 0 is still probably not good to have

[jgraj]

Also there are kinc_affirm and assert calls used seemingly randomly so I didn't know which one to use

[RobDangerous]

kinc_affirm is active in release-builds, assert is not. Choose wisely.