dict_foreign_find_index(): Ignore incompletely created indexes.
After a failed ADD UNIQUE INDEX, an incompletely created index
could be left behind until the next ALTER TABLE statement.
FIXME: MDEV-13668 InnoDB unnecessarily rebuilds table
FIXME: MDEV-13671 InnoDB should use case-insensitive column name comparisons
like the rest of the server
FIXME: MDEV-13640 / Properly fix MDEV-9469 'Incorrect key file' on ALTER TABLE
FIXME: investigate result difference in innodb.innodb-alter-autoinc
and ensure that MariaDB does the right thing with auto_increment_increment
and auto_increment_offset, for both ALGORITHM=INPLACE and ALGORITHM=COPY
(Oracle MySQL behaviour differs between those two).
