This allows to remove special casing for the {count == 0} case, which
was needed because {memmove} does not accept {nullptr} arguments even
if the {count} is zero.

R=leszeks@chromium.org

Bug: v8:9396
Change-Id: Iaef3cdbbffa74c2ba1c4e4501dafd943282cbcd9
Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1807366Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63838}