sharedbook: cast to unsigned to avoid undefined shift

Fixes: Issue 19690: tremor:decode_fuzzer: Undefined-shift in vorbis_book_init_decode

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19619

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/tremor