Description
docker crash

Steps to reproduce the issue:

1. i use fluentd

Describe the results you received:
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: panic: runtime error: invalid memory address or nil pointer dereference
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: [signal SIGSEGV: segmentation violation code=0x1 addr=0x50 pc=0x563fc57ebad9]
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: goroutine 561046 [running]:
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent.(*Fluent).write(0xc000824210, 0xc00153f350, 0xc00153f2f0, 0xc00153f320)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent/fluent.go:384 +0x119
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent.(*Fluent).postRawData(0xc000824210, 0xc00153f350, 0x44, 0x18f43bbd)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent/fluent.go:223 +0x6e
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent.(*Fluent).EncodeAndPostData(0xc000824210, 0xc0005e4b90, 0x44, 0x18f43bbd, 0xed62a3596, 0x0, 0x563fc6792da0, 0xc00153f2f0, 0x98, 0x8)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent/fluent.go:210 +0x1be
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent.(*Fluent).PostWithTime(0xc000824210, 0xc0005e4b90, 0x44, 0x18f43bbd, 0xed62a3596, 0x0, 0x563fc67982c0, 0xc00153f2c0, 0x1a13, 0x563fc41b1833)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/vendor/github.com/fluent/fluent-logger-golang/fluent/fluent.go:201 +0x7bb
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/daemon/logger/fluentd.(*fluentd).Log(0xc001717ec0, 0xc0012ac230, 0x563fc7fa82a0, 0x400a)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/daemon/logger/fluentd/fluentd.go:177 +0x3cb
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: github.com/docker/docker/daemon/logger.(*Copier).copySrc(0xc001576040, 0x563fc5ac17b5, 0x6, 0x7fd920d88d70, 0xc001717f00)
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/daemon/logger/copier.go:123 +0x369
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: created by github.com/docker/docker/daemon/logger.(*Copier).Run
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ dockerd[2120]: /root/rpmbuild/BUILD/src/engine/.gopath/src/github.com/docker/docker/daemon/logger/copier.go:48 +0x111
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ systemd[1]: docker.service: main process exited, code=exited, status=2/INVALIDARGUMENT
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ systemd[1]: Unit docker.service entered failed state.
Apr 16 19:16:38 iZuf6jb9uv1lmingd0agruZ systemd[1]: docker.service failed.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: docker.service holdoff time over, scheduling restart.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Stopped Docker Application Container Engine.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Closed Docker Socket for the API.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Stopping Docker Socket for the API.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Starting Docker Socket for the API.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Listening on Docker Socket for the API.
Apr 16 19:16:40 iZuf6jb9uv1lmingd0agruZ systemd[1]: Starting Docker Application Container Engine...

Describe the results you expected:

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.1
 API version:       1.40
 Go version:        go1.12.5
 Git commit:        74b1e89
 Built:             Thu Jul 25 21:21:07 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.1
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.5
  Git commit:       74b1e89
  Built:            Thu Jul 25 21:19:36 2019
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.6
  GitCommit:        894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc:
  Version:          1.0.0-rc8
  GitCommit:        425e105d5a03fabd737a126ad93d62a9eeede87f
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

 docker info
Client:
 Debug Mode: false

Server:
 Containers: 17
  Running: 15
  Paused: 0
  Stopped: 2
 Images: 59
 Server Version: 19.03.1
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc version: 425e105d5a03fabd737a126ad93d62a9eeede87f
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 3.10.0-957.21.3.el7.x86_64
 Operating System: CentOS Linux 7 (Core)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.638GiB
 Name: iZuf6jb9uv1lmingd0agruZ
 ID: TUDV:GL7K:XGNU:AN7J:Z3X7:KRGN:D2K3:JP5Y:FGVZ:ZOWA:GQR3:7L3G
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Additional environment details (AWS, VirtualBox, physical, etc.):
ALIYUN