Turns out that all callers might need this, so instead of burdening them
with checking, make the helper more robust.

Bug: v8:7151
Change-Id: I54270e2b1f5c03b503e2cf202bcbc09fe91aef14
Reviewed-on: https://chromium-review.googlesource.com/804956Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49844}