For certain invalid meshes (BKE_mesh_is_valid returns false), the
corner space indices might be invalid. However, this is no different
than any other place that code relies on valid mesh inputs; there is
no reason for the face corner normals code alone to bear the burden
of checking for invalid input meshes. Typically we choose not to
slow down the common case or increase its complexity for checks
like this, and the assumption that all corners are in a fan space may
help to simplify this code in the future.
Pull Request: https://projects.blender.org/blender/blender/pulls/145131
24c87d5020