[FIX] mrp: Updating cost from Bom Structure & Cost

Steps to reproduce the bug:

- Let's consider two storable products P1 and P2
- Update the cost of P2 to 10€
- Create a BOM for P1 with P2 as component
- Click on Bom Structure & Cost
- Click on P2
- Try to update the cost

Bug:

A traceback was raised saying that "Record does not exist or has been deleted" or the cost of an other
product was displayed because the active_id was still the id of the bom.

opw:2093199

closes odoo/odoo#39657

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>