Add missing negation to validate the existence of a DEPTH_STENCIL
attachment correctly.

BUG=angleproject:1101
TEST=dEQP-GLES3.functional.state_query.fbo.framebuffer_attachment_x_size_rbo,
     dEQP-GLES3.functional.negative_api.state.get_framebuffer_attachment_parameteriv

Change-Id: I278875684fdff562d16faba9a7eca0aa83bf80e9
Reviewed-on: https://chromium-review.googlesource.com/329867Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>