Check if storage for thread_local variables has been allocated before
attempting to access such variables, as exceptions may be raised in the
thread before this initializion is complete, causing an infinite loop.

Bug: v8:8966
Change-Id: Ifc6223b74999a55bfd0ed2d6ebf054bbffd7e809
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1507714
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60852}