drouting: Fix startup crash with no clustering

The crash was a regression, introduced in 40355a3.

Many thanks to @masudmuborakshohi and Sasmita Panda for the reports!

modules/drouting/dr_clustering.c

@@ -339,6 +339,9 @@ void receive_dr_cluster_event(enum clusterer_event ev, int node_id)
 
 int dr_cluster_sync(void)
 {
+	if (!dr_cluster_id)
+		return 0;
+
 	if (c_api.request_sync(&status_repl_cap, dr_cluster_id) < 0) {
 		LM_ERR("Sync request failed\n");
 		return -1;

modules/drouting/drouting.c

@@ -2030,7 +2030,6 @@ static void rpc_dr_reload_data(int sender_id, void *unused)
 	dr_reload_data(1);
 
 	dr_cluster_sync();
-
 }