fix rdf import warning by turtle/n3 files

If I make an import like this

```
$this->_store->importRdf('http://www.test.de', 'dispediaWebsite_local.n3', $filetype, $locator);
```

I become this warning
```
Notice: Undefined property: Erfurt_Syntax_RdfParser_Adapter_Turtle::$_dataLength in /media/Share/Projekte/Dispedia/OW/libraries/Erfurt/library/Erfurt/Syntax/RdfParser/Adapter/Turtle.php on line 1020
```
because of the protected variable *_dataLength* has not been initialized, I fixed this with commit afc699e, maybe you want to include this changes