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
