Fix race condition issue to solve an issue about "panic: runtime error: invalid memory address or nil pointer dereference". This fix stabilize Docker daemon under the situation of communication problem with Fluentd processes. Signed-off-by: Satoshi Tagomori <tagomoris@gmail.com> Upstream-commit: 87124b9d62bc71f7632126cf7f8d5eb805c4a7f1 Component: engine