avoid shadowing in dig module

```
************* Module salt.modules.dig
salt/modules/dig.py:175: [W0621(redefined-outer-name), NS] Redefining name 'a' from outer scope (line 280)
salt/modules/dig.py:174: [W0621(redefined-outer-name), NS] Redefining name 'ns' from outer scope (line 282)
```