row_upd_changes_field_size_or_external(): Revert some unnecessary
changes, and assert that the update vector does not contain the
magic value UNIV_SQL_DEFAULT.

row_upd_rec_in_place(): Remove a redundant assertion.
rec_set_nth_field() already asserts the same.

row_upd_build_sec_rec_difference_binary(): Assert that there are
no default values in a secondary index tuple.