Fixed an issue with lengths being undefined.

This if-for-if feels convoluted. You could achieve similar results with:
```coffeescript
break if typeof rule.rules is 'undefined'

numNestedRuleSelectors = 0

for nestedRule in rule.rules
  numNestedRuleSelectors += nestedRule.selectors.length
```

We break out of this default case if there aren't any nested rules and reduce the amount of branching and arrowheads. I haven't found a case where we would need to check if `nestedRule.selectors` is undefined, but if you could add that test case that would be awesome.