Fixing an issue in WasmInstanceObject::GetGlobalBufferAndIndex.

Bug: v8:13309
Change-Id: I52e65c109e43ce0300513604e71e587e1949f70a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3907666Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Samuel Groß <saelo@chromium.org>
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#83345}