D3D: Speculative fix for VertexBuffer crash. It's possible we were triggering a null pointer dereference in some cases of lost device recovery. I was unable to reproduce this in a test so this fix is speculative. Bug: chromium:871178 Change-Id: I55b7790eabb81c25489bdfd105b5f2cbeedcc8db Reviewed-on: https://chromium-review.googlesource.com/1169955 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>