fix error handling for nil font_descriptor

In `pdf-reader` the font_descriptor variable is assigned here: [link](https://github.com/yob/pdf-reader/blob/master/lib/pdf/reader/font.rb#L141-L150)

Apparently for CID fonts it will be assigned `nil`. This can cause problems in `extract/font_metrics.rb` when trying to run the `ascent`/`descent` methods on a nil object, and throws this error:

```
pdf-extract-0.1.1/lib/pdf/extract/font_metrics.rb:44:in `initialize': undefined method `ascent' for nil:NilClass (NoMethodError)
```

This will at least allow it to continue running if it encounters a CID font.