dump: Stop scanning at once if list is empty

Empty list on scan error means, that the very top task ran away from
us. There's no point in trying to catch one.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

cr-dump.c

@@ -1036,6 +1036,12 @@ static int collect_pstree(pid_t pid, struct list_head *pstree_list,
 			break;
 		}
 
+		if (list_empty(pstree_list))
+			/*
+			 * No items at all -- no need in re-scanning it again
+			 */
+			break;
+
 		/*
 		 * Old tasks can die and new ones can appear while we
 		 * try to seize the swarm. It's much simpler (and reliable)