In https://chromium-review.googlesource.com/c/v8/v8/+/1866771 we added
a static regexp stack area to ensure a stack always exists. We
apparently forgot to update EnsureCapacity s.t. we skip
dynamically-allocating a stack when the static stack suffices.

Found by lizeb@, thanks!

Bug: v8:11540
Change-Id: Ie63b0b5e5959fbf0768cc3597f63943b1775fbf2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2749015
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73337}