[FIX] Prevent production lot deletion if it is used in a stock moves

https://bugs.launchpad.net/openobject-addons/+bug/1176912
In 'stock.move' model, the field 'prodlot_id' is defined with default 'ondelete' which is 'set null' in Odoo. This commit changes 'ondelete' to 'restirict'.