header_length and payload_length are filled with random data if an
unknown tcode was read from the AR buffer (i.e. if the AR buffer
contained invalid data).

We still need a better strategy to recover from this, but at least
handle_ar_packet now doesn't return out of bound buffer addresses
anymore.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>