Windows can allocate the stack at low addresses. A low-address on-stack
slot (e.g. backing store reference for Blink's on-heap collections) with
a null value would make TryGetCagedHeap falsely think that the slot
resides in a caged heap that starts at a null address.

We will still crash for low-address on-stack slots with non-null
on-stack value, since these cases are not considered valid and should
not happen.

The null value check is added only to Windows. It is not an issue on
other OSes where the stack always resides at high addresses and we
prefer to keep the write barrier as cheap as possible.

Bug: chromium:1230794, chromium:1056170
Change-Id: I07e2d178cd95edba57015d6bc6eb127a443b0589
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3069146
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76069}