innobase_col_to_mysql(): New function, adapted from
row_sel_field_store_in_mysql_format().

innobase_rec_to_mysql(): Correct the function comment, which was still
saying "clustered index record", although we can convert any record.
Make use of innobase_col_to_mysql().  Always call field->reset(),
so that innobase_col_to_mysql() will not have to pad anything.