res_pjsip_pubsub.c: Set dlg_status code instead of sending SIP response.

We should not try to send a SIP response message because we may be
restoring a persistent subscription where we are not responding to a SIP
request.

Change-Id: Id89167ef90320c5563f37e632db0dda6cb9e7dec

res/res_pjsip_pubsub.c

@@ -1263,7 +1263,7 @@ static struct sip_subscription_tree *create_subscription_tree(const struct ast_s
 
 	sub_tree = allocate_subscription_tree(endpoint);
 	if (!sub_tree) {
-		pjsip_endpt_respond_stateless(ast_sip_get_pjsip_endpoint(), rdata, 500, NULL, NULL, NULL);
+		*dlg_status = PJ_ENOMEM;
 		return NULL;
 	}
 	sub_tree->role = AST_SIP_NOTIFIER;