Prevent crash in robust access mode when buffer has no data 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/1238622 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>