Panic in docker input if endpoint is misconfigured

[russorat]

Relevant telegraf.conf:

[[inputs.docker]]	
  ## Docker Endpoint
  ##   To use TCP, set endpoint = "tcp://[ip]:[port]"
  ##   To use environment variables (ie, docker-machine), set endpoint = "ENV"
  ##   exp: unix:///var/run/docker.sock
  endpoint = "localhost"

  ## Set to true to collect Swarm metrics(desired_replicas, running_replicas)
  gather_services = false

  ## Only collect metrics for these containers, collect all if empty
  container_names = []

  ## Containers to include and exclude. Globs accepted.
  ## Note that an empty array for both will include all containers
  container_name_include = []
  container_name_exclude = []

  ## Container states to include and exclude. Globs accepted.
  ## When empty only containers in the "running" state will be captured.
  # container_state_include = []
  # container_state_exclude = []

  ## Timeout for docker list, info, and stats commands
  timeout = "5s"

  ## Whether to report for each container per-device blkio (8:0, 8:1...) and
  ## network (eth0, eth1, ...) stats or not
  perdevice = true

  ## Whether to report for each container total blkio and network stats or not
  total = false
  
  ## Which environment variables should we use as a tag
  ##tag_env = ["JAVA_HOME", "HEAP_SIZE"]
  ## docker labels to include and exclude as tags.  Globs accepted.
  ## Note that an empty array for both will include all labels as tags
  docker_label_include = []
  docker_label_exclude = []

System info:

Telegraf version: master 3c4e737
operating system name: Mac OS 10.14.2
and other relevant details: Docker isn't running

Steps to reproduce:

1. add config for docker
2. ensure docker isn't running (but the address is invalid so it wouldn't matter)
3. see panic

Expected behavior:

telegraf doesn't panic

Actual behavior:

telegraf panics

2019-01-03T20:27:06Z I! Starting Telegraf
2019-01-03T20:27:06Z I! Loaded inputs: inputs.docker inputs.mem inputs.diskio inputs.disk inputs.kernel inputs.processes inputs.swap inputs.system inputs.cpu
2019-01-03T20:27:06Z I! Loaded aggregators:
2019-01-03T20:27:06Z I! Loaded processors:
2019-01-03T20:27:06Z I! Loaded outputs: influxdb_v2
2019-01-03T20:27:06Z I! Tags enabled: host=prod1-rsavage.local
2019-01-03T20:27:06Z I! [agent] Config: Interval:15ms, Quiet:false, Hostname:"prod1-rsavage.local", Flush Interval:10s
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x48e517f]

goroutine 37 [running]:
github.com/influxdata/telegraf/plugins/inputs/docker.(*Docker).gatherInfo(0xc0002e03c0, 0x6108900, 0xc0003aa9e0, 0x0, 0x0)
	/Users/rsavage/go/src/github.com/influxdata/telegraf/plugins/inputs/docker/docker.go:298 +0x12f
github.com/influxdata/telegraf/plugins/inputs/docker.(*Docker).Gather(0xc0002e03c0, 0x6108900, 0xc0003aa9e0, 0x0, 0x0)
	/Users/rsavage/go/src/github.com/influxdata/telegraf/plugins/inputs/docker/docker.go:168 +0x86
github.com/influxdata/telegraf/internal/models.(*RunningInput).Gather(0xc0000bd140, 0x6108900, 0xc0003aa9e0, 0x60faa80, 0xc0000bcc40)
	/Users/rsavage/go/src/github.com/influxdata/telegraf/internal/models/running_input.go:86 +0x6d
github.com/influxdata/telegraf/agent.(*Agent).gatherOnce.func1(0xc00004a240, 0xc0000bd140, 0x6108900, 0xc0003aa9e0)
	/Users/rsavage/go/src/github.com/influxdata/telegraf/agent/agent.go:305 +0x3f
created by github.com/influxdata/telegraf/agent.(*Agent).gatherOnce
	/Users/rsavage/go/src/github.com/influxdata/telegraf/agent/agent.go:304 +0xdc

Additional info:

[Include gist of relevant config, logs, etc.]