Fix crash when pausing XFB then deleting a buffer. Fix is to validate XFB buffer bindings even if we're paused. This is undefined behaviour so we can use any non-crashing solution. Bug: chromium:1305190 Change-Id: Ib95404cdb13adbde7f34d6cc77473a8b3cbf1de7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3522283 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>