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.

src/resource.c

@@ -827,6 +827,8 @@ coap_resource_notify_observers(coap_resource_t *r, const coap_string_t *query) {
     if (!found)
       return 0;
   } else {
+    if ( !r->subscribers )
+      return 0;
     r->dirty = 1;
   }