Blender Git Loki

Fix crashes caused by library placeholders.

This fixes inconsistencies in materials between objects and obdata
due to placeholders generation for missing libdata.

Note that we cannot do that when generating the obdata placeholder,
as not all objects using it might be already loaded...
So this has to be done near the end of the reading/linking process.

Reported here by Blender Studio.

Reviewers: brecht

Subscribers: jbakker, zeddb

Tags: #datablocks_and_libraries

Differential Revision: https://developer.blender.org/D5428