fix assert in make screenshot

after
    commit a46a2577
    Date:   Wed Jun 5 15:24:04 2019 +0200
    improve empty tools::Rectangle (width)

Change-Id: I496d92712aa7627f7d6932d7d01f70a68a1ee781
Reviewed-on: https://gerrit.libreoffice.org/73657Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>

vcl/source/control/button.cxx

@@ -805,7 +805,7 @@ void PushButton::ImplDrawPushButtonContent(OutputDevice* pDev, DrawFlags nDrawFl
     DrawSymbolFlags         nStyle;
 
     if( aInRect.Right() < aInRect.Left() || aInRect.Bottom() < aInRect.Top() )
-        aInRect.SetEmpty();
+        return; // nothing to do
 
     pDev->Push( PushFlags::CLIPREGION );
     pDev->IntersectClipRegion( aInRect );