Blender Git Loki

Action Editor: Preemptive fix for null-pointer dereference

A few weeks ago, I got a random crash while testing som edge cases
(IIRC, it was trying to assign an action with no active object),
which I haven't been able to reproduce since then. This commit though
adds some extra sanity checks here, as a user may try to assign an
action to an animdata block which did not have an action already.