finalize() segfaults if missing metadata

To reproduce:

``` py
zim_creator = ZimCreator(get_zim_name())
zim_creator.add_article(ZimTestArticle())
zim_creator.finalize()
```

```
Traceback (most recent call last):
  File "libzim/libzim.pyx", line 71, in libzim.ZimArticle._get_data
Segmentation fault
```

Also segfaults if updating all but one metadata.