ofz: handle empty Gradient Stops

Change-Id: I7e719b21bd21085ffdd0226a4a25d406a4e69351
Reviewed-on: https://gerrit.libreoffice.org/45617Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

oox/source/drawingml/fillproperties.cxx

@@ -372,7 +372,7 @@ void FillProperties::pushToPropMap( ShapePropertyMap& rPropMap,
                         ::std::swap( aGradient.StartColor, aGradient.EndColor );
                         ::std::swap( nStartTrans, nEndTrans );
                     }
-                    else
+                    else if (!maGradientProps.maGradientStops.empty())
                     {
                         // A copy of the gradient stops for local modification
                         GradientFillProperties::GradientStopMap aGradientStops(maGradientProps.maGradientStops);