Fix zero size buffer robust init crash. 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/513478 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>