After experiencing a device loss, Renderer11::resetDevice can fail to
recreate mDevice, leaving it null. In a following call to
testDeviceLost, mDevice will be dereferenced, causing a segfault.

BUG=angleproject:1457

Change-Id: Ib5be64542929441ceb7088a328c3245ae7e52f17
Reviewed-on: https://chromium-review.googlesource.com/364120Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>