[FIX] portal: avoid errors during autovacuum

When the cron for autovacuum runs, the osv_memory objects are deleted. The portal.wizard.user object has a required field (wizard_id) linked to another osv_memory object (portal.user) which causes a traceback when the cron tries to delete portal.user object before the portal.wizard.user. opw 609918
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64