Access the rendered buffer only if the document exists

When firing the completion event make sure the document exists before
we try to get the rendered buffer that needs to be returned.

Bug: 751377
Change-Id: Id956f5a287b6b91b7bad04d85b6c8f3c32c9470d
Reviewed-on: https://chromium-review.googlesource.com/598727Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491527}

third_party/WebKit/Source/modules/webaudio/OfflineAudioContext.cpp

@@ -353,14 +353,14 @@ void OfflineAudioContext::FireCompletionEvent() {
   // that the context has been closed.
   SetContextState(kClosed);
 
-  AudioBuffer* rendered_buffer = DestinationHandler().RenderTarget();
-
-  DCHECK(rendered_buffer);
-  if (!rendered_buffer)
-    return;
-
   // Avoid firing the event if the document has already gone away.
   if (GetExecutionContext()) {
+    AudioBuffer* rendered_buffer = DestinationHandler().RenderTarget();
+
+    DCHECK(rendered_buffer);
+    if (!rendered_buffer)
+      return;
+
     // Call the offline rendering completion event listener and resolve the
     // promise too.
     DispatchEvent(OfflineAudioCompletionEvent::Create(rendered_buffer));