These slots are invalid and can result in a broken offset when slot index and start of object are equal and are at the beginning of a cell.

Moreover, make DCHECKs CHECKs to catch bugs in the wild.

BUG=chromium:473174
LOG=n

Review URL: https://codereview.chromium.org/1051243004

Cr-Commit-Position: refs/heads/master@{#27602}