D3D11: Fix bug with static vertex attributes. In some specific cases after binding a zero size buffer we could end up trying to use a buffer storage that was no longer valid. Fix this by ensuring we don't flush dirty bits when we have an early exit due to a zero size buffer. Also adds a regression test. Bug: chromium:1107433 Change-Id: I9db560e8dd3699abed2bb7fe6d91060148ba1817 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2314216 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>