InnoDB: Ignoring strange row from mysql.innodb_index_stats after valid DDL.

TEST                                      RESULT   TIME (ms) or COMMENT

--------------------------------------------------------------------------

worker[1] Using MTR_BUILD_THREAD 300, with reserved ports 16000..16019

CREATE TABLE t1 ( col1 INTEGER, col2 INTEGER ) ENGINE = InnoDB;

ALTER TABLE t1 ADD PRIMARY KEY ( col2, col1 ) ;

ALTER TABLE t1 DROP PRIMARY KEY, ADD INDEX ( col2, col1 ) ;

ALTER TABLE t1 ADD PRIMARY KEY ( col2, col1 ) ;

ALTER TABLE t1 ADD PRIMARY KEY ( col2, col1 ) ;

main.strange_row 'innodb'                [ fail ]

        Test ended at 2021-06-17 12:30:27

CURRENT_TEST: main.strange_row

mysqltest: At line 8: query 'ALTER TABLE t1 ADD PRIMARY KEY ( col2, col1 ) ' failed: <Unknown> (2013): Lost connection to server during query

     The crash is caused by changing ib::info to ib::fatal in storage/innobase/dict/dict0stats.cc.

The server error log shows

2021-06-17 12:30:25 0 [Note] /data/Server_bin/bb-10.6-MDEV-25062H_asan/bin/mariadbd: ready for connections.

Version: '10.6.2-MariaDB-debug-log'  socket: '/data/Server_bin/bb-10.6-MDEV-25062H_asan/mysql-test/var/tmp/mysqld.1.sock'  port: 16000  Source distribution

2021-06-17 12:30:26 4 [ERROR] [FATAL] InnoDB: Ignoring strange row from mysql.innodb_index_stats WHERE database_name = 'test' AND table_name = 't1' AND index_name = 'col2' AND stat_name = 'n_diff_pfx03'; because stat_name is out of range, the index has 2 unique columns

bb-10.6-MDEV-25062 commit 3fdd9844f3567f58f60693e4fa8979fa3fefe929

origin/10.6 commit f778a5d5e2aaff7c58000f9e2423a809db14747c