* [http_listener_v2] Stop() succeeds even if fails to start
In cases where the http_listener_v2 plugin config is invalid, when the agent attempts to cleanup by stopping all the inputs, the Stop method here panics as it tries to call listener.Stop() when no listener has been set. This also masks the error message returned from the Start method.
```
> telegraf --test
2020-10-27T12:21:45Z I! Starting Telegraf 1.16.0
2020-10-27T12:21:45Z I! Using config file: /etc/telegraf/telegraf.conf
...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1245130]
goroutine 45 [running]:
github.com/influxdata/telegraf/plugins/inputs/http_listener_v2.(*HTTPListenerV2).Stop(0xc00043e000)
/go/src/github.com/influxdata/telegraf/plugins/inputs/http_listener_v2/http_listener_v2.go:178 +0x30
github.com/influxdata/telegraf/agent.stopServiceInputs(0xc00045e480, 0x5, 0x8)
/go/src/github.com/influxdata/telegraf/agent/agent.go:445 +0x82
github.com/influxdata/telegraf/agent.(*Agent).testRunInputs(0xc000288080, 0x32be8c0, 0xc0000f1f00, 0x0, 0xc00000f480, 0x0, 0x0)
/go/src/github.com/influxdata/telegraf/agent/agent.go:434 +0x1b7
github.com/influxdata/telegraf/agent.(*Agent).test.func4(0xc000057b70, 0xc000288080, 0x32be8c0, 0xc0000f1f00, 0x0, 0xc00000f480)
/go/src/github.com/influxdata/telegraf/agent/agent.go:977 +0x8b
created by github.com/influxdata/telegraf/agent.(*Agent).test
/go/src/github.com/influxdata/telegraf/agent/agent.go:975 +0x352
```
This fixes this issue by checking if the listener has been set before calling listener.Stop.
```
> ./telegraf --config test.conf --test
2020-10-27T12:43:25Z I! Starting Telegraf
2020-10-27T12:43:25Z E! [agent] Starting input inputs.http_listener_v2: listen tcp: address address_without_port: missing port in address
```
* retry CI
35a221018b