Make sure both the fast and slow version return the same value in case
of wrong follow-bit values in the input.

Bug: chromium:1359230, chromium:1360735
Change-Id: Ic65f81109e5bbc288fa41a5540ec7e6cece10ffc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890998
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83307}