Use helper methods introduced in MDEV-7824 all around the code

The patch for MDEV-7824 is going to add a few useful helper methods.
During review, Sergei suggested to use these new methods in the other code parts:

This is ok, but then, please, see what other places in the code you can
change to use your new helpers. In a separate commit, of course. I
suspect there's a lot of code that's doing, like

/* Get the value from default_values */

diff= (my_ptrdiff_t) (orig_field->table->s->default_values-

orig_field->table->record[0]);

orig_field->move_field_offset(diff); // Points now at default_values

if (orig_field->is_real_null())

field->set_null();

else

{

field->set_notnull();

memcpy(field->ptr, orig_field->ptr, field->pack_length());

}

orig_field->move_field_offset(-diff); // Back to record[0]

(this is from create_tmp_table() in sql_select.cc)