Fix parsing errors in ctap_parse_hmac_secret

[stevenwdv]

1. saltEnc type was not checked causing an assertion in cbor_value_copy_byte_string to fail.
   Reproduce using solo-python: Go to HmacSecretExtension in extensions.py in python-fido2 and replace 2: salt_enc by 2: "hey", run solo key challenge-response.

2. Parameters could be omitted by duplicating known keys, causing parsed_count to increase anyway. This causes reading later on of arrays that are still 0.
   Reproduce using solo-python: Go to HmacSecretExtension in extensions.py in python-fido2 and replace 1: key_agreement by -42: True, then go to dump_dict in cbor.py in python-fido2 and replace it by:

def dump_dict(data):
    items = [(encode(k), encode(v)) for k, v in data.items() if k != -42]
    if -42 in data:
        items.append((encode(2), encode(data[2])))
    items.sort(key=_sort_keys)
    return dump_int(len(items), mt=5) + b"".join([k + v for (k, v) in items])

and run solo key challenge-response.
This will duplicate saltEnc(2) while removing keyAgreement(1), so the total number of keys will remain 3, which ctap_parse_hmac_secret will happily accept. You can also choose other parameters to duplicate. We cannot directly add a duplicate key to the dict because Python will ignore it.

Is there maybe somewhere that regression tests for this can be added? Maybe I could add something to solokeys/fido2-tests, if that's the right place for this.