Bug 1274079, ensure that after dispatching beforeprint, we have still valid ContentViewer to print, r=bz

--HG--
extra : rebase_source : c07cf7794d48d4f9b52b2646c18099a33bbea788

layout/base/nsDocumentViewer.cpp

@@ -3728,6 +3728,9 @@ nsDocumentViewer::PrintPreview(nsIPrintSettings* aPrintSettings,
   nsAutoPtr<nsPrintEventDispatcher> beforeAndAfterPrint(
     new nsPrintEventDispatcher(doc));
   NS_ENSURE_STATE(!GetIsPrinting());
+  // beforeprint event may have caused ContentViewer to be shutdown.
+  NS_ENSURE_STATE(mContainer);
+  NS_ENSURE_STATE(mDeviceContext);
   if (!mPrintEngine) {
     mPrintEngine = new nsPrintEngine();