Add nullptr guard for the return value of AllocateBasicChunk.

Bug: chromium:1097502
Change-Id: Ia4642151a119ccabe58d7084077808aac93e5d1c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2257221Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68454}