Verify that `current` always points inside, or just past the end of the
subject string.

Bug: chromium:1083450
Change-Id: I27ba49cbfd0aa93cd2e305efafc23b155c98a49b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207136Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67879}