Blender Git Loki

Fix T49229: ID user decrement error when deleting group instance twice.

Another great example of inconsistency in usercount handling - dupli_group was considered
as refcounted by readfile.c code (and hence by library_query.c one, which is based on it),
but not by editor/BKE_object code, which never increased group's usercount when creating
an instance of it etc.

To be backported to 2.78.