skins2: fix corner case with old skins and fullscreen

For skins without video elements, fullscreen mode could be launched with the inner video kept hidden. Initializing the video control to NULL fixes it.

modules/gui/skins2/src/vout_manager.cpp

@@ -211,6 +211,10 @@ void* VoutManager::acceptWnd( vout_window_t* pWnd )
         // directly attach vout thread to it
         pCtrlVideo->attachVoutWindow( pVoutWindow );
     }
+    else
+    {
+        pVoutWindow->setCtrlVideo( NULL );
+    }
 
     // save vout characteristics
     m_SavedWndVec.push_back( SavedWnd( pWnd, pVoutWindow, pCtrlVideo ) );