windows: Prevent queueing of multiple thread cancel APCs

This avoids any races during cleanup invocation if multiple cancel() requests
come in.

src/libstrongswan/threading/windows/thread.c

@@ -88,10 +88,15 @@ struct private_thread_t {
 	bool cancelability;
 
 	/**
-	 * Has the thread been cancelled
+	 * Has the thread been cancelled by thread->cancel()?
 	 */
 	bool canceled;
 
+	/**
+	 * Did we schedule an APC to docancel()?
+	 */
+	bool cancel_pending;
+
 	/**
 	 * Active condition variable thread is waiting in, if any
 	 */
@@ -357,11 +362,15 @@ METHOD(thread_t, cancel, void,
 	if (this->cancelability)
 	{
 		threads_lock->lock(threads_lock);
+		if (!this->cancel_pending)
+		{
+			this->cancel_pending = TRUE;
 			QueueUserAPC((void*)docancel, this->handle, (uintptr_t)this);
 			if (this->condvar)
 			{
 				WakeAllConditionVariable(this->condvar);
 			}
+		}
 		threads_lock->unlock(threads_lock);
 	}
 }