don't mirror 0, RECT_EMPTY, 0, RECT_EMPTY rectangle

leave it untouched to make gtk3 rtl spin buttons possible
where the additional mirrorValue contains the region
of the spinbuttons to render within this rectangle

Change-Id: I2648d98ed36373c95947cc4d78e2e54797f01155
Reviewed-on: https://gerrit.libreoffice.org/17940Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-on: https://gerrit.libreoffice.org/17942Tested-by: Jenkins <ci@libreoffice.org>

vcl/source/gdi/salgdilayout.cxx

@@ -728,7 +728,9 @@ bool SalGraphics::DrawNativeControl( ControlType nType, ControlPart nPart, const
     if( (m_nLayout & SalLayoutFlags::BiDiRtl) || (pOutDev && pOutDev->IsRTLEnabled()) )
     {
         Rectangle rgn( rControlRegion );
-        mirror( rgn, pOutDev );
+        Rectangle aNull;
+        if (rgn != aNull)
+            mirror(rgn, pOutDev);
         std::unique_ptr< ImplControlValue > mirrorValue( aValue.clone());
         mirror( *mirrorValue, pOutDev );
         bool bRet = drawNativeControl( nType, nPart, rgn, nState, *mirrorValue, aCaption );