Fixes an error where TemplateLiteral printing in --print-ast
would try to read an element beyond the length of a vector.

BUG=v8:7415, chromium:820596
R=adamk@chromium.org, gsathya@chromium.org

Change-Id: Idf9e0da8c165ee62bc1a348a91c2ed5ed798404a
Reviewed-on: https://chromium-review.googlesource.com/957883Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#51857}