The function was using the `BKE_id_material_len_p` with the ID
of the object rather than the object data ID, which is what the
function expects.
Instead make sure to use `BKE_object_material_len_p` to get
the right number of material slots.
Additionally use the correct slot in `BKE_object_material_get`
which is the index of the material + 1.
Pull Request: https://projects.blender.org/blender/blender/pulls/145323
fff8834213