Blender Git Loki

Quick hacky fix for crash on undo (for Grease Pencil)

AnimData is not copied when making duplicating bGPdata for internal usage
(i.e. for the undo buffer); instead, the data was being shared between the
real copies and the undo buffer copies (to save memory - since otherwise
we'd have to have copies of all the animation data floating around).

The fix here clears these refs before trying to free these copies,
making sure that the data won't get freed incorrectly.