ClusterFuzz found another case where "weird" embedder calls can cause
signed integer overflow. This patch fixes the last addition in that
function to use unsigned types.

Bug: chromium:991676
Change-Id: Ia77a12020908de8f0a3bd1be7d3722ba5c5c919b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1743971
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63349}