make WW8ReadFieldParams::GetResult safe

as shown by kde239121-4.doc

Change-Id: I60356e7d76cc271869079f2fffa00e149274a8cc

filter/source/msfilter/util.cxx

@@ -490,7 +490,7 @@ OUString WW8ReadFieldParams::GetResult() const
         return OUString();
     else
     {
-        return nSavPtr == -1 ? aData.copy(nFnd) : aData.copy(nFnd, nSavPtr-nFnd);
+        return nSavPtr < nFnd ? aData.copy(nFnd) : aData.copy(nFnd, nSavPtr-nFnd);
     }
 }