Fix T61051: crash with multi-object mixed mode snapping
rBec3357e03ab1 introduced multi-object snapping.
Seems like this was done without mixed-mode selections in mind.
So code assumes that all selected objects are actually armatures [which can fail].
In 2.7 this was not a problem, because code only took active object into account, 2.8 was iterating over all selected_editable_objects.
Now just iterate over objects in posemode instead