[Bug] Telegraf crashes when upgrading mongodb

[wei-hai]

Relevant telegraf.conf:

# Read metrics from one or many MongoDB servers
[[inputs.mongodb]]
  servers = ["mongodb://127.0.0.1:27017"]
  gather_perdb_stats = true

System info:

ubuntu 12.04
telegraf 1.3.3
mongodb 2.0.4

Steps to reproduce:

1. start telegraf and mongodb
2. run apt-get update to upgrade mongodb to latest version

Expected behavior:

telegraf should not crash and reconnect after mongodb upgrade

Actual behavior:

telegraf crashes

Additional info:

telegraf.log

2017-07-11T23:22:05Z E! Error in plugin [inputs.mongodb]: no reachable servers
2017-07-11T23:23:05Z E! Error in plugin [inputs.mongodb]: no reachable servers
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc5b950]

goroutine 5765 [running]:
github.com/influxdata/telegraf/plugins/inputs/mongodb.NewStatLine(0xed0f7545c, 0x2d9af9c, 0x1bec540, 0xc4202a02d0, 0xc42087dae0, 0xc4208d3cf8, 0xc4201207a0, 0xed0f75510, 0x3208e81, 0x1bec540, ...)
	/home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongostat.go:517 +0x340
github.com/influxdata/telegraf/plugins/inputs/mongodb.(*Server).gatherData(0xc420382940, 0x1b03540, 0xc4203828e0, 0x406001, 0x0, 0x0)
	/home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb_server.go:107 +0xd0f
github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).gatherServer(0xc4202a6080, 0xc420382940, 0x1b03540, 0xc4203828e0, 0x1145a00, 0xc4207ac540)
	/home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:171 +0x89
github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather.func1(0xc4207d9380, 0x1b03540, 0xc4203828e0, 0xc4202a6080, 0xc420382940)
	/home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:99 +0x73
created by github.com/influxdata/telegraf/plugins/inputs/mongodb.(*MongoDB).Gather
	/home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/mongodb/mongodb.go:100 +0x3ec