Fix some more undefined behaviour

As https://github.com/xiph/flac/issues/784 notified me of
undefined behaviour going undetected by the fuzzers, I ran the test
suite with the full undefined behaviour sanitizer enabled.

There is one bit of undefined behaviour that I cannot fix,
FLAC__metadata_get_picture allows a picture type of -1 to mean all,
but this isn't part of the enum.