We were not correctly checking for a zero-size buffer, which should
not trigger allocating a scratch buffer.

BUG=angleproject:1635

Change-Id: Iac0a3d8f1eeeb7b39c59eec411bec78a4f21bcd3
Reviewed-on: https://chromium-review.googlesource.com/513478Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>