Using that idiom on an empty vector results in an undefined behavior
that I have seen cause a vector to get length -1 with clang 3.6 on
Linux.

BUG=

Change-Id: I3bb8d8884efa29a17672b458263067644dbf0fec
Reviewed-on: https://chromium-review.googlesource.com/286740Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>