Fix VideoEncoder null deref crash

CreateAcceleratedVideoEncoder() returns a nullptr on failure. We need
to check |media_encoder_| before initializing it.

Bug: 1123080
Change-Id: I244c29f85602097a8348c037a0a2a3555c1f12f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382012
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802813}

third_party/blink/renderer/modules/webcodecs/video_encoder.cc

@@ -477,6 +477,12 @@ void VideoEncoder::ProcessConfigure(Request* request) {
       break;
   }
 
+  if (!media_encoder_) {
+    // CreateAcceleratedVideoEncoder() can return a nullptr.
+    HandleError(DOMExceptionCode::kOperationError, "Encoder creation error.");
+    return;
+  }
+
   frame_size_ = gfx::Size(config->options.width, config->options.height);
 
   auto output_cb = WTF::BindRepeating(&VideoEncoder::MediaEncoderOutputCallback,