In part_verify_links() complain if a DM particle has an ID or neg. offset of 0.

src/part.c

@@ -119,7 +119,7 @@ void part_verify_links(struct part *parts, struct gpart *gparts,
     if (gparts[k].type == swift_type_dark_matter) {
 
       /* Check that it's not linked */
-      if (gparts[k].id_or_neg_offset < 0)
+      if (gparts[k].id_or_neg_offset <= 0)
         error("DM gpart particle linked to something !");
     }