MDEV-16499 [10.1] ER_NO_SUCH_TABLE_IN_ENGINE followed by "Please drop the table and recreate" upon adding FULLTEXT key to table with virtual column

There was an incorrect check for MariaDB and InnoDB
tables fields count. Corruption was reported when there was no corruption.
Also, a warning message had incorrect field numbers for both MariaDB and InnoDB
tables.

ha_innobase::open(): fixed check and message