.. to avoid the expensive malloc call.

Fixed: v8:9455
Change-Id: I6734fe07a3884b228d818f60be83d9e45c2ee383
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3190105
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77118}