The D3D11 backend used to crash to a null pointer dereference if a
glClearBuffer call was done on a nonexistent depth or stencil
attachment. Validate for these conditions so that the backend can't
crash in this case.
BUG=angleproject:2827
TEST=angle_end2end_tests
Change-Id: Iecee78d213d11d492d52f246b4b068e8b6f34244
Reviewed-on: https://chromium-review.googlesource.com/1233675Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
