copy_data_between_tables() sets to->s->default_fields to 0, as a part
of the code disabling ON UPDATE actions for all old fields
(so ON UPDATE is enable only for new fields during copying).
After the actual copying, copy_data_between_tables() did not restore
to->s->default_fields to the original value. As a result, the
TABLE_SHARE to->s was left in a wrong state after copy_data_between_tables()
and further open_table_from_share() using this TABLE_SHARE did not
populate TABLE::default_field, which further made
TABLE::evaluate_update_default_function() crash on access to NULL
pointer.

Fix:
Changing copy_data_between_tables() to restore to->s->default_fields
to its original value after the copying loop.