Do not preinitialize the feedback vector slot if
the lookup iterator is an interceptor, because it is not
guaranteed that the iterator has a PropertyCell.

If the HandlerConfiguration has a non-masking
intercepting setter,
the iterator does not have a valid PropertyCell.

BUG=chromium:656648

Review-Url: https://codereview.chromium.org/2674103002
Cr-Commit-Position: refs/heads/master@{#43034}