de_skolemize fails when handling literals

`Graph.de_skolemize()` incorrectly tries to deskolemize literals, which fails in some edge cases, like below:

```python
from rdflib import Graph

nt = '<http://example.com> <http://example.com> "http://example.com [some remark]" .'
graph = Graph().parse(data=nt, format="nt").de_skolemize()
```

Which raises a `ValueError` with message `"Invalid IPv6 URL"`.