network: add assertions

As they do not have default callback functions.

src/network/networkd-address.c

@@ -570,6 +570,7 @@ int address_configure(
         assert(link->ifindex > 0);
         assert(link->manager);
         assert(link->manager->rtnl);
+        assert(callback);
 
         /* If this is a new address, then refuse adding more than the limit */
         if (address_get(link, address->family, &address->in_addr, address->prefixlen, NULL) <= 0 &&

src/network/networkd-route.c

@@ -513,6 +513,7 @@ int route_configure(
         assert(link->manager->rtnl);
         assert(link->ifindex > 0);
         assert(IN_SET(route->family, AF_INET, AF_INET6));
+        assert(callback);
 
         if (route_get(link, route->family, &route->dst, route->dst_prefixlen, route->tos, route->priority, route->table, NULL) <= 0 &&
             set_size(link->routes) >= routes_max())