Underflow not detected for range length field

[senier]

The following spec is excepted even though Len can become negative:

package Example is                                                                                                                                                            

   type Len is range 40 .. 1500 with Size => 16;

   type TLV is
      message
         Len : Len
            then Data with Length => 8 * (Len - 50);
         Data : Opaque;
      end message;
end Example;