cid#1606629 Overflowed constant

Change-Id: Id0438b5ae8abdba4d9a87584f9586b8a53852ac6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/174852
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sw/source/core/layout/paintfrm.cxx

@@ -741,6 +741,7 @@ void SwLineRects::ConnectEdges( OutputDevice const *pOut, SwPaintProperties cons
                                                       SubColFlags::Tab);
                             if ( isFull() )
                             {
+                                assert(i > 0);
                                 --i;
                                 k = aCheck.size();
                                 break;