Bug 1568655. Handle null return value from nsGlobalWindowInner::GetScriptableTop in ImageCacheKey::GetTopLevelBaseDomain. r=aosmond

Every other call to this function in the tree null checks the return value.

Differential Revision: https://phabricator.services.mozilla.com/D39243

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

image/ImageCacheKey.cpp

@@ -194,7 +194,7 @@ nsCString ImageCacheKey::GetTopLevelBaseDomain(Document* aDocument,
   if (!AntiTrackingCommon::MaybeIsFirstPartyStorageAccessGrantedFor(
           aDocument->GetInnerWindow(), aURI)) {
     nsPIDOMWindowOuter* top = aDocument->GetInnerWindow()->GetScriptableTop();
-    nsPIDOMWindowInner* topInner = top->GetCurrentInnerWindow();
+    nsPIDOMWindowInner* topInner = top ? top->GetCurrentInnerWindow() : nullptr;
     if (!topInner) {
       return aDocument
           ->GetBaseDomain();  // because we don't have anything better!