Validate non-negative vertex attribute offsets. When robust buffer access is enabled, validat that buffers cannot be read with a negative index. Bug: chromium:323699974 Change-Id: I05beb72c60832ab4decbf16c47b3d92bd374286b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5435736 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>