Handle nullptr mSwapChain in SwapChain11::getSyncValues There are 6 crashes in SwapChain11::getSyncValues on crash server. I looked at one of them and it is due to a nullptr mSwapChain which appears to be a valid state that is handled in other SwapChain11 methods. This change adds a similar check / returning an error to SwapChain11::getSyncValues. BUG=angleproject:1402 Change-Id: I9c17b884704b8aaaddf2d802911cd0ca961ae0b4 Reviewed-on: https://chromium-review.googlesource.com/402613 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Stanislav Chiknavaryan <stanisc@chromium.org>