Gtid_log_event Construction from File Should Ensure Event Length When Using Extra Flags

The extra flags check and actions when constructing a GTID log event during replication, i.e. the constructor invoked in log_event.cc when constructed from binlog content, currently checks that the true length of the event is consistent with the expectation that is set by each flag using DBUG_ASSERT() invocations. Instead, this check should be coded into the conditionals themselves, to ensure that we don't accidentally read memory beyond the end of the event, if the flag and data aren't consistent.