This function didn't account for addresses of api-provided external
references, leading to out-of-bound reads on external_reference_table.
(This happened to me when printing a code object in gdb, I'm not sure
how to easily test it.)

Also remove an unused method from the private Value class.

R=jgruber@chromium.org

Change-Id: Id14fed3fb3866df750bcad8f4a02c61748b07ad3
Reviewed-on: https://chromium-review.googlesource.com/1060035Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53195}