resource.c: Return 0 if there is no observer to notify.
resource.c: Return 0 if there is no observer to notify.

According to documentation, coap_resource_notify_observers() must return 0 if
no observer will be notified. This can help an application release resources
associated to that resource for observation.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap