If `out` is empty accessing `out.back()` is invalid.

TBR=yangguo@chromium.org

Bug: chromium:894934
Change-Id: I7286c5b6a9857f1cdb2bcaf383094bee65bac393
Reviewed-on: https://chromium-review.googlesource.com/c/1282565Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56669}