obj: Fix dereference before NULL check

The check for !obj indicates that obj might be NULL, thus move the call
to obj_ops(obj) - which dereferences obj - after the check.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>