Fix possible logical replication crash.

Commit c3afe8cf5a added a new
password_required option but forgot that you need database access
to check whether an arbitrary role ID is a superuser.

Report and patch by Hou Zhijie. I added a comment. Thanks to
Alexander Lakhin for devising a way to reproduce the crash.

Discussion: http://postgr.es/m/OS0PR01MB5716BFD7EC44284C89F40808948F9@OS0PR01MB5716.jpnprd01.prod.outlook.com

src/backend/replication/logical/worker.c

@@ -4545,12 +4545,14 @@ ApplyWorkerMain(Datum main_arg)
 		replorigin_session_setup(originid, 0);
 		replorigin_session_origin = originid;
 		origin_startpos = replorigin_session_get_progress(false);
-		CommitTransactionCommand();
 
 		/* Is the use of a password mandatory? */
 		must_use_password = MySubscription->passwordrequired &&
 			!superuser_arg(MySubscription->owner);
 
+		/* Note that the superuser_arg call can access the DB */
+		CommitTransactionCommand();
+
 		LogRepWorkerWalRcvConn = walrcv_connect(MySubscription->conninfo, true,
 												must_use_password,
 												MySubscription->name, &err);