Bug 749964. Don't try to save if what we have is not a real document object. r=khuey

embedding/components/webbrowserpersist/src/nsWebBrowserPersist.cpp

@@ -1517,6 +1517,9 @@ nsresult nsWebBrowserPersist::SaveDocumentInternal(
 
     // Persist the main document
     nsCOMPtr<nsIDocument> doc(do_QueryInterface(aDocument));
+    if (!doc) {
+        return NS_ERROR_UNEXPECTED;
+    }
     mURI = doc->GetDocumentURI();
 
     nsCOMPtr<nsIURI> oldBaseURI = mCurrentBaseURI;