Skip headers that can not be split

Fixes #11955 

How to test:
Click New -> from Link
Add a link. e.g.: http://www.stadtwolke.de/m/f9104abd.png
Check owncloud.log:
```Debug PHP Undefined offset: 1 at /apps/files/ajax/newfile.php#xyz```
will appear. Exact line number depends on OC version