The problem is hard to repeat, and I failed to create a deterministic
test case. Online index creation creates stubs for to-be-created indexes.
If index creation fails, we could remove these stubs while locks exist
in the indexes. (This would require that the index creation was completed,
and a concurrent DML operation acquired a lock on a record in the
uncommitted index. If a duplicate key error occurs in an uncommitted
index, the error will be reported for the CREATE UNIQUE INDEX, not for
the DML operation that tried to insert the duplicate.)

dict_table_try_drop_aborted(), row_merge_drop_indexes(): If transactional
locks exist on the table, keep the table->indexes intact.