This fixes a crash appearing in robust access mode when a buffer has
no data by stopping the draw call.

A better fix would be to ensure that fetches from an empty buffer
result in zero, but this fix requires a lot more effort. This is not
necessarily warranted since the client is either way doing something
incorrect in this case.

BUG=angleproject:2840
TEST=angle_end2end_tests

Change-Id: Ie4ef5b1ca7c0091d7d26f91678d6e4768cc0c650
Reviewed-on: https://chromium-review.googlesource.com/1238622Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>