Vulkan: Fix XFB invalid accesses in buffer OOM. This uses the "null" buffer in the Renderer to bind an empty buffer handle so ANGLE can maintain a consistent state. Bug: chromium:1086532 Change-Id: I1912a1d1cb64433a285fcfced80a675619690a0b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2219140 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>