Add missing buffer validation to BeginTransformFeedback. A change to Vulkan exposed this missing validation. In the Vulkan back-end we do some caching on BeginXFB, which would perform an invalid memory access. Adding the missing validation correctly traps the error before we reach the back-end. Bug: chromium:1171685 Bug: angleproject:4622 Change-Id: I5c92575a07149e431c4f260a4555ff196822c64e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2937022 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>