skins2: correct crashes when using zoom and multiple video tags

Fixes #2821
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/gui/skins2/controls/ctrl_video.cpp

@@ -222,6 +222,8 @@ void CtrlVideo::detachVoutWindow( )
 
 void CtrlVideo::resizeInnerVout( )
 {
+    if( m_pVoutWindow )
+    {
         WindowManager &rWindowManager =
              getIntf()->p_sys->p_theme->getWindowManager();
         TopWindow* pWin = getWindow();
@@ -233,5 +235,6 @@ void CtrlVideo::resizeInnerVout( )
 
         rWindowManager.show( *pWin );
         m_pVoutWindow->show();
+    }
 }