`CBORParser` need to validate zero-length byte[] for `BigInteger`

[cowtowncoder]

(from https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32250)

Note: similar to #257, CBOR parser should have special handling for case of trying to construct BigInteger from 0-length byte array. Seems reasonable to just construct BigInteger.ZERO for such case.