AqHBCI: Fixed a problem with ChallengeHHD.
- some banks send HHD code with a length field size of 3 digits, some use
only 2 bytes. This can't be detected from outside, so we need to check first
for a three byte size and if that fails we fallback to 2 bytes length.
Should work with any ChallengeHHD now.
At least does work with Hamburger Sparkasse.
- check total length of ChallengeHHF code