ofz#9908 validate no negative editengine selection positions

Change-Id: I37dace2051518224c55756362facd3edba4a9571
Reviewed-on: https://gerrit.libreoffice.org/59058
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/filter/ww8/ww8graf.cxx

@@ -582,7 +582,9 @@ namespace
     bool IsValidSel(const EditEngine& rEngine, const ESelection& rSel)
     {
         const auto nParaCount = rEngine.GetParagraphCount();
-        return rSel.nStartPara < nParaCount && rSel.nEndPara < nParaCount;
+        if (rSel.nStartPara < nParaCount && rSel.nEndPara < nParaCount)
+            return rSel.nStartPos >= 0 && rSel.nEndPos >= 0;
+        return false;
     }
 }