Check for null pointers so DisplayOzone::terminate() doesn't crash when
DisplayOzone::initialize() wasn't completely successful.

BUG=angleproject:1419

Change-Id: I70e994490a7282083d5289e6c35b077676688b65
Reviewed-on: https://chromium-review.googlesource.com/354700Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>