sw: fix ASAN detected error in RtfSdrExport::Commit()

EscherPropertyContainer may allocate a 1-byte buffer if there are no
vertices / segments. (happens in sw_filters_test)

 in EscherPropertyContainer::CreateCustomShapeProperties(MSO_SPT, com::sun::star::uno::Reference<com::sun::star::drawing::XShape> const&) filter/source/msfilter/escherex.cxx:3853:25

Change-Id: I56dedbd57d38cd017183060e924cb1340ea58ebf

sw/source/filter/ww8/rtfsdrexport.cxx

@@ -259,7 +259,8 @@ void RtfSdrExport::Commit(EscherPropertyContainer& rProps, const Rectangle& rRec
             EscherPropSortStruct aSegments;
 
             if (rProps.GetOpt(ESCHER_Prop_pVertices, aVertices) &&
-                    rProps.GetOpt(ESCHER_Prop_pSegmentInfo, aSegments))
+                rProps.GetOpt(ESCHER_Prop_pSegmentInfo, aSegments) &&
+                aVertices.nPropSize >= 6 && aSegments.nPropSize >= 6)
             {
                 const sal_uInt8* pVerticesIt = aVertices.pBuf + 6;
                 sal_Size nVerticesPos = 6;