fallback to 'default :inherited face

I'm getting a whole bunch of 
```
Invalid face reference: nil
Invalid face attribute :inherit nil
```
in my messages buffer.

This patch fixes it (after refreshing the tree) but maybe the correct fix is to call a fontifying function (`jit-lock-fontify-now` was mentioned on the [wiki page][1]).

[1]: http://www.emacswiki.org/SimpleCallTree