InnoDB fails to find the foreign key index for the foreign key relation
in the table while iteration the foreign key constraints.
This is caused by commit 5f09b53bdb4e973e7c7ec2c53a24c98321223f98 (MDEV-31086).

Test case to repeat the issue:

--source include/have_innodb.inc

SET FOREIGN_KEY_CHECKS=0;

CREATE TABLE t1 (f1 INT NOT NULL, INDEX(f1)) ENGINE=InnoDB;

CREATE TABLE t2(f1 INT NOT NULL, f2 VARCHAR(100) DEFAULT NULL,

                INDEX idx(f1, f2),

                FOREIGN KEY(f1) REFERENCES t1(f1))ENGINE=INNODB;

ALTER TABLE t2 DROP INDEX idx;

ALTER TABLE t2 MODIFY f2 VARCHAR(1023);

DROP TABLE t2, t1;

Fix:
===
In check_col_is_in_fk_indexes(), while iterating through
the foreign key relationship, InnoDB should consider that
foreign key relation may not have foreign index when
foreign key check is disabled.