[FIX] core: assignment of binary field on new record with origin

The assignment of a binary field on a new record with origin did
actually modify the attachment storing the value of the field on the
origin record!  The fix is simply to avoid messing up with attachments
when updating new records.

closes odoo/odoo#59278

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>