Currently we don't check the result of AllocateUninitialized and just
continue execution, failing at the following memcpy.

Instead, check for a nullptr result from the embedder-provided allocator
and crash with OOM.

Bug: chromium:853706
Change-Id: I7a9b4e5d0ce3ef2b2cbb5fd3dc949ec2705bdfaa
Reviewed-on: https://chromium-review.googlesource.com/1113454Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54127}