monitor process exited by signal SIGKILL, clean cgroup resource by third party

src/lxc/cgroups/cgfsng.c

@@ -1098,11 +1098,13 @@ __cgfsng_ops static void cgfsng_monitor_destroy(struct cgroup_ops *ops,
 			goto try_recursive_destroy;
 		}
 
+		if (handler->monitor_pid != 0) {
 			ret = lxc_write_openat(pivot_path, "cgroup.procs", pidstr, len);
 			if (ret != 0) {
 				SYSWARN("Failed to move monitor %s to \"%s\"", pidstr, pivot_path);
 				continue;
 			}
+		}
 
 try_recursive_destroy:
 		ret = recursive_destroy(h->monitor_full_path);