D3D: Fix buffer overflow in VertexBuffer.cpp. Under certain situations an integer overflow could lead to ANGLE writing to places where it shouldn't. BUG=518206 Change-Id: I9217685daecb160a4072fbf79c26e5bee9f4621e Reviewed-on: https://chromium-review.googlesource.com/292391 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>