should check pcntl_wifexited first before pcntl_wexitstatus

[Jinglever]

In lib/Worker.php line:215 in Function work(): I think it should check pcntl_wifexited first before pcntl_wexitstatus. Because pcntl_wexitstatus is meaningless if pcntl_wifexited return false. For example, kill the child process would result in this case.