Bug fix for a crash, when any_errors_fatal is true

Reported by Rumen:

TASK: [fail FAIL] *************************************************************
skipping: [hostname.com]
failed: [hostname.com] => {"failed": true}
msg: Failed as requested from task
Traceback (most recent call last):
  File "/usr/local/bin/ansible-playbook", line 268, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/usr/local/bin/ansible-playbook", line 208, in main
    pb.run()
  File "/Library/Python/2.7/site-packages/ansible/playbook/__init__.py", line 262, in run
    if not self._run_play(play):
  File "/Library/Python/2.7/site-packages/ansible/playbook/__init__.py", line 580, in _run_play
    if (hosts_count - len(host_list)) > int((play.max_fail_pct)/100.0 * hosts_count):
TypeError: object of type 'NoneType' has no len()