Otherwise when trying to use a DEPTH texture for DEPTH_STENCIL,
attach(nullptr) gets called but the FramebufferAttachment still returns
true when isAttached is called.

BUG=angleproject:1523

Change-Id: I30b78aff619eb6cd63e0befac886bddc177a2e58
Reviewed-on: https://chromium-review.googlesource.com/418403
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>