Blender Git Loki

Fix T49553: Blender 2.78 crashes when File->Data Previews ->Refresh Datablock Previews

New recursive iteration over IDs in BKE_library_foreach_ID_link() was
broken by the infamous nodetree case. We cannot really recusively call
this function in that case, so better to deffer handling of
non-datablock NodeTrees as if real IDs here.

Also fixed initial ID not being stored as handled, in rare cases this
could also lead to infinite looping.

To be backported to 2.78a.