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

Problem:
========
SHOW BINLOG EVENTS FROM <pos> reports following ASAN error

AddressSanitizer: SEGV on unknown address
The signal is caused by a READ memory access.
User_var_log_event::User_var_log_event(char const*, unsigned int,
    Format_description_log_event const*)

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

Fix:
===
**Part8: added checks to avoid reading out of buffer limits**

sql/log_event.cc

@@ -7957,6 +7957,11 @@ User_var_log_event(const char* buf, uint event_len,
       we keep the flags set to UNDEF_F.
     */
     uint bytes_read= ((val + val_len) - buf_start);
+    if (bytes_read > event_len)
+    {
+      error= true;
+      goto err;
+    }
     if ((data_written - bytes_read) > 0)
     {
       flags= (uint) *(buf + UV_VAL_IS_NULL + UV_VAL_TYPE_SIZE +