MDEV-18788 Live upgrade from MySQL 5.6/5.7 to MariaDB 10.4 fails with "Event...

MDEV-18788 Live upgrade from MySQL 5.6/5.7 to MariaDB 10.4 fails with "Event Scheduler: An error occurred when initializing system tables"

if columns or indexes are modified/renamed/dropped in an ALTER TABLE,
stat tables must be updated accordingly (e.g. all statistics for a column
should be dropped). But if a stat table doesn't exist, it's not a reason
to fail the whole ALTER TABLE operation - such an error should be ignored.

mysql-test/main/stat_tables_missing.result

+create table t1 (a int);
+alter table mysql.column_stats rename to mysql.column_stats1;
+flush tables;
+alter table t1 change a b varchar(100);
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `b` varchar(100) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+alter table mysql.column_stats1 rename to mysql.column_stats;
+drop table t1;

mysql-test/main/stat_tables_missing.test

+#
+# MDEV-18788 Live upgrade from MySQL 5.6/5.7 to MariaDB 10.4 fails with "Event Scheduler: An error occurred when initializing system tables"
+#
+create table t1 (a int);
+alter table mysql.column_stats rename to mysql.column_stats1;
+flush tables;
+alter table t1 change a b varchar(100);
+show create table t1;
+alter table mysql.column_stats1 rename to mysql.column_stats;
+drop table t1;

sql/sql_statistics.cc

@@ -273,7 +273,11 @@ static inline int open_stat_table_for_ddl(THD *thd, TABLE_LIST *table,
                                          Open_tables_backup *backup)
 {
   table->init_one_table(&MYSQL_SCHEMA_NAME, stat_tab_name, NULL, TL_WRITE);
-  return open_system_tables_for_read(thd, table, backup);
+  No_such_table_error_handler nst_handler;
+  thd->push_internal_handler(&nst_handler);
+  int res= open_system_tables_for_read(thd, table, backup);
+  thd->pop_internal_handler();
+  return res;
 }