COW is generating NULL active edit bones, which is fixed with this patch.
This way, ED_object_editmode_calc_active_center returns the correct active
edit bone.

However, cow needs to be updated when I change the active object. Otherwise
it gives me the wrong values until I go in and out of the edit mode.

What do you think Sergey?

The alternative is to not use the evaluated obedit
but the orignial one in snap_calc_active_center. It would fix the problem,
but I'm yet to investigate why we are using the evaluatd object there in the
first place (on rBe12df10120a0).