[Eva] Fixes the evaluation engine when a domain oracle returns Bottom.

When the evaluation of any subexpression leads to Bottom, the evaluation of the
complete expression returns only Bottom with no valuation: thus the valuation
cache is no longer updated.

However, when the evaluation of another expression requested by a domain through
the oracle leads to Bottom, the evaluation of the primary expression may
continue without leading to Bottom (domains are allowed to request the evaluation
of 1/0 at any time, after all).

In this case, we should properly update the cached valuation by binding Bottom
to the expression requested by the domain, in case it should be used again in
the evaluation (for instance if the requested evaluation was a subexpression of
the primary target of the evaluation).

And especially, if subdivided evaluations are enabled, the reference to the
cached valuation must _always_ be reset after the oracle (even when it returns
Bottom), as the cache has been used for successive evaluations with various
hypotheses, and the last one is probably not sound in the general case.