We need to properly handle the case of uninitialized embedder data slots
which contain the "undefined" value and thus might look like valid
external pointer table indices.

Bug: v8:10391
Change-Id: I169a3e42132dde223ea151c1a5d5956c72341f8d
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3448378Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79009}