fix fdo#60910 FILESAVE loses fontwork/shape objects

This reinstates one line of code which was removed by
7fec8dfc.

Change-Id: Ie2af33914d17a46b7118b8b804aadc0aa3bc5d90
Reviewed-on: https://gerrit.libreoffice.org/3596Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>

svx/source/svdraw/svdoashp.cxx

@@ -3217,6 +3217,7 @@ bool SdrObjCustomShape::doConstructOrthogonal(const OUString& rName)
 void SdrObjCustomShape::InvalidateRenderGeometry()
 {
     mXRenderedCustomShape = 0L;
+    mxCustomShapeEngine = 0L;
     SdrObject::Free( mpLastShadowGeometry );
     mpLastShadowGeometry = 0L;
 }