Cleanup: remove runtime pointer from workob in what_does_obaction

Any access to this after calling what_does_obaction would reference
invalid stack memory with undefined behavior.
Set to null to ensure this never happens.

source/blender/blenkernel/intern/action.cc

@@ -1779,6 +1779,8 @@ void what_does_obaction(Object *ob,
     /* execute effects of Action on to workob (or its PoseChannels) */
     BKE_animsys_evaluate_animdata(&workob->id, &adt, anim_eval_context, ADT_RECALC_ANIM, false);
   }
+  /* Ensure stack memory set here isn't accessed later, see !118847. */
+  workob->runtime = nullptr;
 }
 
 void BKE_pose_check_uids_unique_and_report(const bPose *pose)