MDEV-3798: [SHOW] EXPLAIN UPDATE/DELETE, Memory leak in binlog.binlog_base64_flag:

- It turns out, there are statements that will call lex_start(thd->lex) 
  after parsing has been finished. lex_start() will set lex->explain=NULL,
  which will lose the pointer to already allocated Explain_plan object.
- To get rid of this, switch to lazy creation of lex->explain.  Now, it is 
  created only when we get a part ot query plan.

sql/sql_parse.cc

@@ -2201,8 +2201,6 @@ mysql_execute_command(THD *thd)
     thd->mdl_context.release_transactional_locks();
   }
   
-  create_explain_query(thd->lex, thd->mem_root);
-
 #ifndef DBUG_OFF
   if (lex->sql_command != SQLCOM_SET_OPTION)
     DEBUG_SYNC(thd,"before_execute_sql_command");