Problem:
========
InnoDB fails to find the foreign key index for the
foreign key relation in the table while iterating the
foreign key constraints during alter operation. This is
caused by commit 5f09b53b
(MDEV-31086).

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.