nstat plugin does not run properly without explicit paths

[rjjanuary]

Bug report

Relevant telegraf.conf:

Working config:
[[inputs.nstat]]
proc_net_netstat = "/proc/net/netstat"
proc_net_snmp = "/proc/net/snmp"
proc_net_snmp6 = "/proc/net/snmp6"
dump_zeros = true

Configs resulting in runtime errors below:
[[inputs.nstat]]
dump_zeros = true

[[inputs.nstat]]
proc_net_netstat = ""
proc_net_snmp = ""
proc_net_snmp6 = ""
dump_zeros = true

System info:

-bash-4.2$ telegraf -version
Telegraf - version 0.13.1
-bash-4.2$ cat /etc/redhat-release
CentOS Linux release 7.2.1511 (Core)
-bash-4.2$ uname -a
Linux ets-vvaprd-metrics-a01.cdc.nicusa.com 3.10.0-327.4.5.el7.x86_64 #1 SMP Mon Jan 25 22:07:14 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Steps to reproduce:

1. include sample nstat influx config into telegraf.conf
2. restart

Expected behavior:

nstat plugin collects info as documented

Actual behavior:

telegraf process produces runtime errors

Additional info:

Error from telegraf.log when not supplying explicit paths in telegraf config

panic: runtime error: index out of range

goroutine 15 [running]:
panic(0x107ebc0, 0xc820010060)
        /usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/influxdata/telegraf/plugins/inputs/nstat.loadUglyTable(0xc8205ad000, 0xbd4, 0xe00, 0x1, 0x0, 0x0, 0x0)
        /home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/nstat/nstat.go:197 +0x83c
github.com/influxdata/telegraf/plugins/inputs/nstat.(*Nstat).gatherSNMP(0xc82015ab80, 0xc8205ad000, 0xbd4, 0xe00, 0x7f06aaf8f9a0, 0xc820209fb0, 0x0, 0x0)
        /home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/nstat/nstat.go:111 +0x5f
github.com/influxdata/telegraf/plugins/inputs/nstat.(*Nstat).Gather(0xc82015ab80, 0x7f06aaf8f9a0, 0xc820209fb0, 0x0, 0x0)
        /home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/plugins/inputs/nstat/nstat.go:81 +0x1d9
github.com/influxdata/telegraf/agent.gatherWithTimeout.func1(0xc820058c00, 0xc82000f440, 0xc820209fb0)
        /home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/agent/agent.go:174 +0x73
created by github.com/influxdata/telegraf/agent.gatherWithTimeout
        /home/ubuntu/telegraf-build/src/github.com/influxdata/telegraf/agent/agent.go:175 +0xe0