Blender Git Loki

Fix T49273: Crash during access to dupli weights at launch time.

See commit's comments for details, but this boils down to: do not try to use
purely runtime cache data as a 'real' ID pointer in readcode, it's likely
doomed to fail in some cases, and is bad practice in any case!

Thix fix implies dupliweight's object will be invalid until first scene update
(i.e. first particles evaluation).