If a different object was active, clicking on a linked armature's pose
in the Outliner would enter Pose Mode for it.
This would actually cause a failed assert, but in release builds the
armature would just enter pose mode.
Steps to reproduce were:
* Link in armature object
* Activate a different object
* In the Outliner, un-collapse the armature object
* Activate Pose Mode by clicking on its pose there
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