Blender Git Loki

Fix T89033: segfault reordering animation channels

Fix segmentation fault that can occur when reordering animation
channels.

Under some specific conditions, the list "act->curves" is empty in the
"join_groups_action_temp" function. In particular, this happens when a
scene contains an action that has not been pushed down, and with no
keyframe in it.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11569