协程无法正常切回call_user_func

## 环境信息
```
[root@Shimmer ~]# cat /etc/issue
CentOS release 6.8 (Final)
Kernel \r on an \m
[root@Shimmer ~]# cat /proc/version
Linux version 2.6.32-642.11.1.el6.x86_64 (mockbuild@c1bm.rdu2.centos.org) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-17) (GCC) ) #1 SMP Fri Nov 18 19:25:05 UTC 2016
[root@Shimmer ~]# php -v
PHP 7.1.2 (cli) (built: Feb 25 2017 13:04:59) ( ZTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.2, Copyright (c) 1999-2017, by Zend Technologies
[root@Shimmer ~]# php -i | grep 'Configure Command'
Configure Command =>  './configure'  '--prefix=/usr/local/php' '--enable-inline-optimization' '--disable-debug' '--disable-rpath' '--enable-shared' '--enable-opcache' '--enable-fpm' '--with-fpm-user=nginx' '--with-fpm-group=nginx' '--with-mysql=mysqlnd' '--with-mysqli=mysqlnd' '--with-pdo-mysql=mysqlnd' '--with-gettext' '--enable-mbstring' '--with-iconv=/usr/local' '--with-mcrypt' '--with-mhash' '--with-openssl' '--enable-bcmath' '--enable-soap' '--with-libxml-dir' '--enable-pcntl' '--enable-shmop' '--enable-sysvmsg' '--enable-sysvsem' '--enable-sysvshm' '--enable-sockets' '--with-curl' '--with-zlib' '--enable-zip' '--with-bz2' '--with-readline' '--with-gd' '--enable-maintainer-zts'
[root@Shimmer ~]#php -i | grep swoole
swoole
swoole support => enabled
swoole.aio_thread_num => 2 => 2
swoole.display_errors => On => On
swoole.message_queue_key => 0 => 0
swoole.unixsock_buffer_size => 8388608 => 8388608
swoole.use_namespace => On => On
[root@Shimmer ~]# php -r "var_dump(SWOOLE_VERSION);"
string(5) "2.0.6"
```
## 测试代码

链接：https://eyun.baidu.com/s/3mhZj0WK 密码：hdKM

## 描述

执行`php start.php`后，访问`127.0.0.1:9501`，控制台输出内容如下：
```
[root@Shimmer ~]# php start.php
CALL_USER_FUNC START
```
call_user_func后面的代码均没有执行

![代码](http://git.oschina.net/uploads/images/2017/0312/192142_c8da55d3_10096.jpeg "代码")

GVP swoole/swoole-src

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GVPswoole/swoole-src

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