box: assert that auth salt size is correct

Authentication salt is expected to be 20 byte long (SCRAMBLE_SIZE).
However, IPROTO sends 36 bytes (IPROTO_SALT_SIZE). Let's add a few
assertions ensuring that we never pass less than 20 bytes to
authentication methods.

Part of #7987

NO_DOC=code cleanup
NO_TEST=code cleanup
NO_CHANGELOG=code cleanup