fix(kmod): fix is_subscriber_referencing bug

Signed-off-by: veqcc <ryuta.kambe@tier4.jp>

kmod/agnocast.c

@@ -259,9 +259,9 @@ static struct publisher_info * insert_publisher_info(
 
 static bool is_subscriber_referencing(struct entry_node * en)
 {
-  // Since referencing_subscriber_ids always stores entries in order from the lowest index,
-  // if there's nothing at index 0, it means it doesn't exist.
-  return (en->referencing_subscriber_ids[0] > 0);
+  // The referencing_subscriber_ids array is always populated starting from the smallest index.
+  // Therefore, an empty element at index 0 is equivalent to a non-existent referencing subscriber.
+  return (en->referencing_subscriber_ids[0] != -1);
 }
 
 static void remove_referencing_subscriber_by_index(struct entry_node * en, int index)