Check that a register used as a local is within the bytecode array's
local count.

BUG=chromium:706234

Change-Id: I51f6a0a8be065b93b9a4e1dca623e98c51685b51
Reviewed-on: https://chromium-review.googlesource.com/464768Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44308}