Bug 1505494 - not allow autoplay if we don't have a document. r=cpearce

In order to avoid the nullptr crash when using `approver`.

Differential Revision: https://phabricator.services.mozilla.com/D11501

--HG--
extra : moz-landing-system : lando

dom/media/AutoplayPolicy.cpp

@@ -91,6 +91,10 @@ IsWindowAllowedToPlay(nsPIDOMWindowInner* aWindow)
   }
 
   nsIDocument* approver = ApproverDocOf(*aWindow->GetExtantDoc());
+  if (!approver) {
+    return false;
+  }
+
   if (nsContentUtils::IsExactSitePermAllow(approver->NodePrincipal(),
                                            "autoplay-media")) {
     AUTOPLAY_LOG("Allow autoplay as document has autoplay permission.");