FORMAT_DESCRIPTION_LOG_EVENT::CALC_SERVER_VERSION_SPLIT

Problem: When reading a Format_description_log_event, it supposes MySQL
version is always valid and DBUG_ASSERTION is used check the version number.
However, user may give a wrong binlog offset, even give a faked binary event
which includes an invalid MySQL version. This will cause server crash.

Fix: The assertions are removed and an error will be reported if MySQL
version in Format_description_log_event is invalid.