Incorrect reads of frozen binlog events; FDE corrupted in relay log

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.

Patch is forthcoming.