Fix Bug in thread initialisation: only init hash if USERPROG is defined

src/threads/thread.c

@@ -238,7 +238,10 @@ thread_create (const char *name, int priority,
   struct thread *parent_thread = thread_current ();
   init_thread (t, name, parent_thread->nice, priority, parent_thread->recent_cpu);
   tid = t->tid = allocate_tid ();
+
+  #ifdef USERPROG
     hash_init (&t->open_files, fd_hash, fd_less, NULL);
+  #endif
 
   /* Prepare thread for first run by initializing its stack.
      Do this atomically so intermediate values for the 'stack'