bug 304896: Null check mNavigator when we set it, since it isn't guaranteed to be non-null. r+sr=jst

dom/src/base/nsGlobalWindow.cpp

@@ -948,9 +948,11 @@ nsGlobalWindow::SetNewDocument(nsIDOMDocument* aDocument,
         mInnerWindowHolder = wsh->GetInnerWindowHolder();
         mNavigator = wsh->GetNavigator(); // This assignment addrefs.
 
+        if (mNavigator) {
           // Update mNavigator's docshell pointer now.
           mNavigator->SetDocShell(mDocShell);
           mNavigator->LoadingNewDocument();
+        }
       } else {
         if (thisChrome) {
           newInnerWindow = new nsGlobalChromeWindow(this);