The problem was that copying a CDDerivedMesh (CDDM_copy) doesn't
copy the vert_normals reference that it takes from a mesh. Since this
entire area is almost completely broken anyway (mainly in terms of
ownership handling), for now we can just avoid copying the DerivedMesh
in the zero levels case.

Longer term, this area should be refactored to remove DerivedMesh
and use the newer subdivision evaluation system.