Fix index expression in interpretert::evaluate

Using integer2string here lead to a value having bad widths.

Author: Joel Allred

src/goto-programs/interpreter_evaluate.cpp

@@ -931,7 +931,7 @@ void interpretert::evaluate(
         if(idx.size() == 1)
         {
           evaluated_index.op1() =
-            constant_exprt(integer2string(idx[0]), expr.op1().type());
+            from_integer(idx[0], expr.op1().type());
         }
         simplified = simplify_expr(evaluated_index, ns);
       }