Move the nexus.IsUninitialized() check into ExtractReceiverMaps so that
we don't need to duplicate the bailout.

Change-Id: I55bdb9baca22f25f681a7a32f4ec56d599c748c6
Reviewed-on: https://chromium-review.googlesource.com/903169Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51112}