Problem:
========
SHOW BINLOG EVENTS FROM <pos> reports following assert when ASAN is enabled.

Rows_log_event::Rows_log_event(const char*, uint,
    const Format_description_log_event*):
Assertion `var_header_len >= 2'

Implemented part of upstream patch.
commit: mysql/mysql-server@a3a497ccf7ecacc900551fb1e47ea4078b45c351

Fix:
===
**Part2: Avoid reading out of buffer limits**