fix invalid string access in OUString::copy

Change-Id: Idccec35f2838b54a7915aa4a0aeb229c69bae74d

sc/source/core/tool/rangeutl.cxx

@@ -396,7 +396,7 @@ void ScRangeStringConverter::GetTokenByOffset(
         sal_Unicode cQuote)
 {
     sal_Int32 nLength = rString.getLength();
-    if( nOffset >= nLength )
+    if( nOffset == -1 || nOffset >= nLength )
     {
         rToken = OUString();
         nOffset = -1;