Blender Git Loki

Fix error when filtering in Outliner "Blender File" mode with libraries

Steps to reproduce were:
* Load factory settings
* Link any library
* Change to "Blender File" display mode in Outliner
* Enable filtering
* -> Assert fails

This may have had further side-effects for linked IDs.
Checked with Bastien, the NULL-check seems reasonable. It was initially there
but removed in d74f9c4b7b4f.

Fine to backport.