When using trap handlers, memory references do not get any checks inserted. This
means there is no check for a null memory as happens when the memory size is
0. Normally this would be correctly caught as an out of bounds access, since the
low memory addresses are not normally mapped. However, if they were mapped for
some reason, we would not catch the out of bounds access.

The fix is to ensure WebAssembly instances always have a guard region even if
the memory is size 0.

This is a rewrite of 5e76ff5a

Note that this can lead to a large amount of unnecessary address space usage,
so we share a single reservation for empty array buffers.

Bug: chromium:769637

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ia8e84be6d595e347d3d342959f2c374db1a3f683
Reviewed-on: https://chromium-review.googlesource.com/702657Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52163}