[FIX] models: avoid neverending recomputation on missing records

Assume F and G are computed by the same method on a missing record R.
During recomputation of F on R, the compute method is called but fails
because R is missing.  Both fields are re-marked to compute (because
computation failed), then F is discarded (because R is missing).  Then
comes G's turn: G is accessed on R and the computation fails.  Both
fields are re-marked to compute (because computation failed), then G is
discarded (because R is missing).  Now F is marked again to compute: the
process never ends.

To avoid this situation, discard all fields to recompute on missing
records.

closes odoo/odoo#42202

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>