explicit thread_exit () when process_start () fails

src/userprog/process.c

@@ -188,6 +188,7 @@ start_process (void *proc_start_data)
 fail:
   data->success = false;
   sema_up (&data->loaded);
+  thread_exit ();
 }
 
 /* Helper function that initializes the stack of a newly created