Kernel: Dequeue dying threads from WaitQueue

If a thread is waiting but getting killed, we need to dequeue
the thread from the WaitQueue so that a potential wake before
finalization doesn't happen.

Kernel/Thread.h

@@ -521,6 +521,7 @@ public:
 private:
     IntrusiveListNode m_runnable_list_node;
     IntrusiveListNode m_wait_queue_node;
+    WaitQueue* m_wait_queue { nullptr };
 
 private:
     friend class SchedulerData;