Implement robust the GetBufferPointerv entry point. BUG=angleproject:1354 Change-Id: Id7dd8438224adb1e2729bcdc18a306e5dfc83a3b Reviewed-on: https://chromium-review.googlesource.com/401399 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>