Do not crash when skipping async tasks

##### Issue Type: Bugfix Pull Request
##### Ansible Version: 1.6.3, devel
##### Environment: doesn't matter
##### Summary:
This will fix gh-7432. 

Git bisect indicates that this was originally broken [here](https://github.com/ansible/ansible/commit/e09313120c96c00980aef12c3105647bdc3857ca), where previously the job id was always set to None initially. I am unsure of the semantics of  `self.runner.vars_cache`, so I did the simpler (but perhaps less robustly correct) thing.
##### Steps To Reproduce:
```
---
- hosts: localhost
  tasks:
  - action: shell /bin/true 
    async: 15
    poll: 0
    when: False
```
##### Expected Results:
Nothing happens
##### Actual Results:
Ansible crashes