We recently changed uc32 to be an unsigned type, and with the invalid
marker being static_cast<uc32>(-1) this DCHECK no longer holds. After
this CL it expicitly checks for the invalid marker.

Bug: v8:10568,chromium:1094226
Change-Id: Idd9efe055b38387e3e37b132cb786cca130767b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2245592
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68333}