After a failed ADD INDEX, dict_index_remove_from_cache_low()
could iterate the index fields and dereference a freed virtual
column object when trying to remove the index from the v_indexes
of the virtual column.

This regression was caused by a merge of
MDEV-16119 InnoDB lock->index refers to a freed object.

ha_innobase_inplace_ctx::clear_added_indexes(): Detach the
indexes of uncommitted indexes from virtual columns, so that
the iteration in dict_index_remove_from_cache_low() can be avoided.

ha_innobase::prepare_inplace_alter_table(): Ignore uncommitted
corrupted indexes when rejecting ALTER TABLE. (This minor bug was
revealed by the extension of the test case.)

dict_index_t::detach_columns(): Detach an index from virtual columns.
Invoked by both dict_index_remove_from_cache_low() and
ha_innobase_inplace_ctx::clear_added_indexes().

dict_col_t::detach(const dict_index_t& index): Detach an index from
a column.

dict_col_t::is_virtual(): Replaces dict_col_is_virtual().

dict_index_t::has_virtual(): Replaces dict_index_has_virtual().