MDEV-18046: Assortment of crashes, assertion failures and ASAN errors in mysql_show_binlog_events

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**

sql/log_event.cc

@@ -9546,7 +9546,14 @@ Rows_log_event::Rows_log_event(const char *buf, uint event_len,
       which includes length bytes
     */
     var_header_len= uint2korr(post_start);
-    assert(var_header_len >= 2);
+    /* Check length and also avoid out of buffer read */
+    if (var_header_len < 2 ||
+        event_len < static_cast<unsigned int>(var_header_len +
+          (post_start - buf)))
+    {
+      m_cols.bitmap= 0;
+      DBUG_VOID_RETURN;
+    }
     var_header_len-= 2;
 
     /* Iterate over var-len header, extracting 'chunks' */