dispatch_semaphore崩溃

当某个dispatch_semaphore_t正处于dispatch_semaphore_wait的时候，释放这个dispatch_semaphore_t的引用(sema = nil)，将会导致崩溃。
示例：

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.sema = dispatch_semaphore_create(0);
    
    // 每隔1秒释放一个信号量
    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        dispatch_semaphore_signal(self.sema);
    }];
    
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 模拟10个任务
        for (NSInteger index = 1; index <= 10; ++ index) {
            // 每个任务都需要等待到有信号量才能开始
            dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER);
            NSLog(@"执行任务%@...", @(index));
        }
        
        // 全部任务执行完毕
        [timer invalidate];
    });
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"touchesEnded");
    
    // crash!
    // 当某个dispatch_semaphore_t正处于dispatch_semaphore_wait的时候
    // 释放这个dispatch_semaphore_t的引用，将会导致崩溃
    self.sema = nil;
}

控制台：

2016-11-22 16:01:50.520 SemaDemo[2990:136781] 执行任务1...
2016-11-22 16:01:51.482 SemaDemo[2990:136781] 执行任务2...
2016-11-22 16:01:52.495 SemaDemo[2990:136781] 执行任务3...
2016-11-22 16:01:53.209 SemaDemo[2990:136725] touchesEnded