snmp: with no mibs installed, snmp panics

[powersj]

Relevent telegraf.conf

[[inputs.snmp]]
  agents = [ "synology.local:161" ]
  version = 2
  timeout = "5s"
  retries = 3
  community = "public"
  name = "system"

  [[inputs.snmp.field]]
      name = "hostname"
      oid = "SNMPv2-MIB::sysName.0"
      is_tag = true

  [[inputs.snmp.table]]
    name = "synotable"
    oid = "SYNOLOGY-DISK-MIB::diskTable"
    inherit_tags = [ "hostname" ]

  [[inputs.snmp.table.field]]
    name = "diskID"
    oid = "SYNOLOGY-DISK-MIB::diskID"
    is_tag = true

[[outputs.file]]

System info

master

Docker

No response

Steps to reproduce

On a system with no snmp installed, telegraf panics when trying to use an snmp plugin.

Expected behavior

Error message about no mibs found, no panic

Actual behavior

2021-12-17T20:23:41Z I! Starting Telegraf 1.22.0-94cf4952
2021-12-17T20:23:41Z I! Loaded inputs: snmp
2021-12-17T20:23:41Z I! Loaded aggregators: 
2021-12-17T20:23:41Z I! Loaded processors: 
2021-12-17T20:23:41Z I! Loaded outputs: file
2021-12-17T20:23:41Z I! Tags enabled: host=ryzen
2021-12-17T20:23:41Z I! [agent] Config: Interval:10s, Quiet:false, Hostname:"ryzen", Flush Interval:10s
2021-12-17T20:23:41Z D! [agent] Initializing plugins
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x2c1a2b3]

goroutine 1 [running]:
github.com/influxdata/telegraf/internal/snmp.LoadMibsFromPath.func1({0x4e080ff, 0x14}, {0x0, 0x0}, {0x0, 0x2c19ac0})
	/home/powersj/telegraf/internal/snmp/translate.go:60 +0x53
path/filepath.Walk({0x4e080ff, 0x14}, 0xc00161f640)
	/usr/lib/go/src/path/filepath/path.go:503 +0x50
github.com/influxdata/telegraf/internal/snmp.LoadMibsFromPath({0xc00040e960, 0x1, 0xc00161f6c8}, {0x586ae70, 0xc000a53d40})
	/home/powersj/telegraf/internal/snmp/translate.go:57 +0x29a
github.com/influxdata/telegraf/plugins/inputs/snmp.(*Snmp).Init(0xc00061bce0)
	/home/powersj/telegraf/plugins/inputs/snmp/snmp.go:102 +0x50
github.com/influxdata/telegraf/models.(*RunningInput).Init(0xc00161f758)
	/home/powersj/telegraf/models/running_input.go:82 +0x35
github.com/influxdata/telegraf/agent.(*Agent).initPlugins(0xc0007280c8)
	/home/powersj/telegraf/agent/agent.go:189 +0x96
github.com/influxdata/telegraf/agent.(*Agent).Run(0xc0007280c8, {0x5802308, 0xc000d616c0})
	/home/powersj/telegraf/agent/agent.go:105 +0x185
main.runAgent({0x5802308, 0xc000d616c0}, {0x83ae658, 0x0, 0x0}, {0x83ae658, 0x0, 0x0})
	/home/powersj/telegraf/cmd/telegraf/telegraf.go:312 +0xc57
main.reloadLoop({0x83ae658, 0x0, 0x0}, {0x83ae658, 0x0, 0x0})
	/home/powersj/telegraf/cmd/telegraf/telegraf.go:147 +0x28a
main.run(...)
	/home/powersj/telegraf/cmd/telegraf/telegraf_posix.go:8
main.main()
	/home/powersj/telegraf/cmd/telegraf/telegraf.go:485 +0xa9a

Additional info

No response