media/gpu/vaapi: Fix crash when destroying VaapiVD before initialization.

This CL fixes a crash in the VaapiVideoDecoder that occurs when the video
decoder is destroyed before it has been initialized.

TEST=ran new VD tests on eve

BUG=991064

Change-Id: I8d2f31e8ed0cdf19de7d8eb537078b09ed19c706
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737442
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685117}

media/gpu/vaapi/vaapi_video_decoder.cc

@@ -243,9 +243,11 @@ void VaapiVideoDecoder::Destroy() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(client_sequence_checker_);
   VLOGF(2);
 
-  decoder_thread_task_runner_->PostTask(
-      FROM_HERE, base::BindOnce(&VaapiVideoDecoder::DestroyTask, weak_this_));
-  decoder_thread_.Stop();
+  if (decoder_thread_task_runner_) {
+    decoder_thread_task_runner_->PostTask(
+        FROM_HERE, base::BindOnce(&VaapiVideoDecoder::DestroyTask, weak_this_));
+    decoder_thread_.Stop();
+  }
 
   delete this;
   VLOGF(2) << "Destroying VAAPI VD done";