Fix handling of messages with single opaque field

[treiher]

package Test is

   type Message is
      message
         Data : Opaque;
      end message;

   generic
      Channel : Channel with Readable, Writable;
   session Session with
      Initial => Start,
      Final => Terminated
   is
      M_R : Message; 
      M_S : Message; 
   begin
      state Start is
      begin
         Channel'Read (M_R); 
      transition
         goto Process 
      end Start;

      state Process is
      begin
         M_S := Message'(Data => M_R.Data);
      transition
         goto Reply 
      exception
         goto Terminated 
      end Process;

      state Reply is
      begin
         Channel'Write (M_S); 
      transition
         goto Terminated 
      end Reply;

      state Terminated is null state; 
   end Session;

end Test;

input:
  Channel:
    - 1 2 3
output:
  - Channel
sequence: |
  Write Channel: 1 2 3
  State: Start
  State: Process
  Read Channel: 1 2 3
  State: Reply

E           AssertionError: non-zero exit status 1
E           Write Channel: 1 2 3
E           State: Start
E           State: Process
E           
E           raised CONSTRAINT_ERROR : rflx-test-message.ads:574 range check failed