MDEV-10775 System table in InnoDB format allowed in MariaDB could lead to crash

when opening a system table for a SELECT-like read, pretend
(for the sake of engines) it's SQLCOM_SELECT

mysql-test/suite/innodb/r/system_tables.result

+alter table mysql.time_zone_name engine=InnoDB;
+create table envois3 (starttime datetime) engine=InnoDB;
+insert envois3 values ('2008-08-11 22:43:00');
+select convert_tz(starttime,'UTC','Europe/Moscow') starttime from envois3;
+starttime
+2008-08-12 02:43:00
+drop table envois3;
+alter table mysql.time_zone_name engine=MyISAM;

mysql-test/suite/innodb/t/system_tables.test

+--source include/have_innodb.inc
+
+#
+# MDEV-10775 System table in InnoDB format allowed in MariaDB could lead to crash
+#
+alter table mysql.time_zone_name engine=InnoDB;
+create table envois3 (starttime datetime) engine=InnoDB;
+insert envois3 values ('2008-08-11 22:43:00');
+--source include/restart_mysqld.inc
+select convert_tz(starttime,'UTC','Europe/Moscow') starttime from envois3;
+drop table envois3;
+alter table mysql.time_zone_name engine=MyISAM;

sql/sql_base.cc

@@ -9223,6 +9223,7 @@ open_system_tables_for_read(THD *thd, TABLE_LIST *table_list,
   */
   lex->reset_n_backup_query_tables_list(&query_tables_list_backup);
   thd->reset_n_backup_open_tables_state(backup);
+  thd->lex->sql_command= SQLCOM_SELECT;
 
   if (open_and_lock_tables(thd, table_list, FALSE,
                            MYSQL_OPEN_IGNORE_FLUSH |