For a locked shapekey, a SculptSession's orig_cos / deform_cos /
deform_imats are not initialized (they only are when
deform_modifiers_active is true -- this in turn is only true for
shapekeys if they are //not// locked [and for deforming modifiers of
course])
We can just update that keyblock with sculpt_update_keyblock() in case
of a locked shapekey
Maniphest Tasks: T79622
Differential Revision: https://developer.blender.org/D8499
21118fb610