[FIX] website_slides: fix slide.slide statistics compute method

The slide.slide and slide.channel "_compute_slides_statistics" methods do not
always assign a default value for the "total_slides" field.

This can cause some CacheMiss and seem related to nondeterministic errors on
the runbot.

Task#2123711
Runbot build error ID 247

closes odoo/odoo#41797

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>