Fix process_result locking by acquiring in process_wait as well to prevent freeing memory too early

src/threads/thread.c

@@ -647,6 +647,7 @@ init_process_result (struct thread *t)
   struct process_result *result = malloc (sizeof (struct process_result));
   result->tid = t->tid;
   result->exit_status = t->exit_status;
+  lock_init (&result->lock);
   sema_init (&result->sema, 0);
   t->result = result;
 }