Fix crash

```
./omq amqp -t /queue --amqp-subject foo -C 1
```
crashed with
```
reason: {badarg,
            [{erlang,list_to_binary,
                 [undefined],
             {rabbit_amqp_session,ensure_terminus,5,
                 [{file,"rabbit_amqp_session.erl"},{line,2046}]},
             {rabbit_amqp_session,ensure_target,3,
                 [{file,"rabbit_amqp_session.erl"},{line,1705}]},
             {rabbit_amqp_session,handle_control,2,
                 [{file,"rabbit_amqp_session.erl"},{line,715}]},
             {rabbit_amqp_session,handle_cast,2,
                 [{file,"rabbit_amqp_session.erl"},{line,331}]},
             {gen_server,try_handle_cast,3,
                 [{file,"gen_server.erl"},{line,1121}]},
             {gen_server,handle_msg,6,
                 [{file,"gen_server.erl"},{line,1183}]},
             {proc_lib,init_p_do_apply,3,
                 [{file,"proc_lib.erl"},{line,241}]}]}
```

Author: ansd

deps/rabbit/src/rabbit_amqp_session.erl

@@ -2042,6 +2042,10 @@ ensure_terminus(target, {topic, _bindingkey}, _, _, _) ->
 ensure_terminus(source, {topic, _BindingKey}, Vhost, User, Durability) ->
     %% exchange amq.topic exists
     declare_queue(generate_queue_name(), Vhost, User, Durability);
+ensure_terminus(target, {queue, undefined}, _, _, _) ->
+    %% Target "/queue" means publish to default exchange with message subject as routing key.
+    %% Default exchange exists.
+    undefined;
 ensure_terminus(_, {queue, QNameList}, Vhost, User, Durability) ->
     declare_queue(list_to_binary(QNameList), Vhost, User, Durability);
 ensure_terminus(_, {amqqueue, QNameList}, Vhost, _, _) ->

deps/rabbit/test/amqp_system_SUITE_data/fsharp-tests/Program.fs

@@ -374,6 +374,7 @@ module Test =
              "/exchange/amq.fanout",  "/exchange/amq.fanout/",  "",  true
              "/exchange/amq.direct",  "/exchange/amq.direct/",  "",  true
              "/exchange/amq.direct",  "/exchange/amq.direct/a", "a", true
+             "/queue",  "/queue/b", "b", true
 
              (* FIXME: The following three tests rely on the queue "test"
               * created by previous tests in this function. *)