This includes several changes:
- avoid a very-unlikely-but-theoretically-possible OOB write
- avoid a somewhat-likely memory leak
- grow the buffer less aggressively for medium-length strings

Change-Id: I877f43d7e2e7cd4778ba8c7c7525ba988301f750
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3771900Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81808}