Currently several places use description_event->common_header_len instead of
  LOG_EVENT_MINIMAL_HEADER_LEN when parsing events with "frozen" headers (such
  as Start_event_v3 and its subclasses such as Format_description_log_event, as
  well as Rotate_event). This causes events with extra headers (which would otherwise
  be valid and those headers ignored) to be corrupted due to over-reading or skipping
  into the data portion of the log events.
  
  It is rewritten in some details patch of Jeremy Cole (See MDEV):
  - The virtual function returns length to avoid IFs (and only one call of the virtual function made)
  - Printing function avoids printing strings