[Zion] Don't try to set state on dying threads in cleanup.

zion/object/thread.cpp

@@ -120,8 +120,10 @@ void Thread::Cleanup() {
   // 2. Unblock waiting threads.
   while (blocked_threads_.size() != 0) {
     auto thread = blocked_threads_.PopFront();
-    thread->SetState(Thread::RUNNABLE);
-    gScheduler->Enqueue(thread);
+    if (!thread->IsDying()) {
+      thread->SetState(Thread::RUNNABLE);
+      gScheduler->Enqueue(thread);
+    }
   }
 
   // 3. Release Kernel Stack