[heap] Fix DCHECK in UndoLastAllocationAt

Change-Id: I4ec51c8f1a0a9b5d9967898c2e89c9ba82ace6c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2997107


Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75485}