[FIX] res.users: safer read() when computing default company value

The goal is to avoid any kind of prefetching
of other fields when copmuting the default
company, as this will sometimes happen in the
middle of a database update, when the default
company is used for setting the default value
of new columns. In that case the db schema
may not fully match the in-memory model, leading
to an SQL error.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0