MDEV-16961 Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' failed upon concurrent DELETE and DDL with virtual blob column

After iterating all fields and setting PART_INDIRECT_KEY_FLAG as
necessary, TABLE::mark_columns_used_by_virtual_fields() remembers
in TABLE_SHARE that this operation was done and need not be repeated.

But as the flag is set in TABLE_SHARE, PART_INDIRECT_KEY_FLAG must
be set in TABLE_SHARE::field[], not only in TABLE::field[].

Otherwise all new TABLEs opened from this TABLE_SHARE will
never have it.