s->iothread is checked for NULL on object creation in colo_compare_complete,
so it's guaranteed not to be NULL.
This resolves a false alert from Coverity (CID 1429969).

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>