Also validate 'defaults' in states.file.managed

Prevents ugly backtraces which don't really point out the actual issue like this one:
```
        Comment: An exception occurred in this state: Traceback (most recent call last):
                   File "salt/state.py", line 1379, in call
                    File "salt/states/file.py", line 1246, in managed
                    File "salt/modules/file.py", line 2363, in check_managed
                    File "salt/modules/file.py", line 2099, in get_managed
                    File "salt/utils/templates.py", line 53, in render_tmpl
                  ValueError: dictionary update sequence element #0 has length 1; 2 is required
```