Calibration HTR: fix of Unpacker

When previous version of Unpacker has been used (as a private branch in
 71X) for current MWGR data in private RAW data unpacking, a problem of the
 unpacker hanging and memory blowing has been observed in some (rare)
 events. It was traced down to improper handling of the corrupted HTR
 calibration data fragments.

This is the fix suggested by Jeremy Mans for 72X soon coming as online
 release.