[21.05.3-2] Start panic with InfluxDB exporter

[albinpopote]

Describe the bug
Harvest seems to launch Prometeus exporter even if it is not configured on harvest.xml and panic

Environment

• Harvest version: harvest version 21.05.3-2 (commit b482aff) (build date 2021-06-28T21:07:14+0530) linux/amd64
• Command line arguments used: systemctl start harvest or bin/harvest start --config harvest.yml
• OS: Debian 10.9
• Install method: apt from DEB package
• ONTAP Version: 9.5

To Reproduce
Configure the YAML configuration file with only InfluxDB as exporter

Expected behavior
Harvest should launch and polling

Actual behavior
Got a panic message

Datacenter            Poller                PID        PromPort        Status
+++++++++++++++++++++ +++++++++++++++++++++ ++++++++++ +++++++++++++++ ++++++++++++++++++++
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x717f9b]

goroutine 1 [running]:
goharvest2/pkg/conf.loadPrometheusExporterPortRangeMapping()
        goharvest2/pkg/conf/conf.go:261 +0xdb
goharvest2/pkg/conf.GetPrometheusExporterPorts(0xc000195b28, 0xd, 0xd, 0xc000195b28, 0xd)
        goharvest2/pkg/conf/conf.go:201 +0x607
main.getPollerPrometheusPort(0xc0001687e0, 0xbf2060, 0xc0000b9580)
        command-line-arguments/harvest.go:565 +0x77
main.doManageCmd(0xc00015b900, 0xc0000d0c40, 0x0, 0x2)
        command-line-arguments/harvest.go:199 +0x9bc
github.com/spf13/cobra.(*Command).execute(0xc00015b900, 0xc0000d0c20, 0x2, 0x2, 0xc00015b900, 0xc0000d0c20)
        github.com/spf13/cobra@v1.1.3/command.go:856 +0x2c2
github.com/spf13/cobra.(*Command).ExecuteC(0xbf6000, 0x0, 0xbb5768, 0xc00008e058)
        github.com/spf13/cobra@v1.1.3/command.go:960 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/cobra@v1.1.3/command.go:897
main.main()
        command-line-arguments/harvest.go:679 +0x34

Additional context
YAML configuration file:

Exporters:
    my_influxdb:
        exporters: InfluxDB
        addr: -REDACTED-
        bucket: NetApp
        org: OBS
        token: -REDACTED-
Defaults:
    collectors:
        - ZapiPerf
    use_insecure_tls: true
Pollers:
    DC-01:
        datacenter: Secret
        addr: -REDACTED-
        auth_style: basic_auth
        username: -REDACTED-
        password: -REDACTED-
        exporters:
            - my_influxdb

[cgrinds]

Thanks for the clear reproduction steps @albinpopote. 👍 Fixed by #286

[albinpopote]

Tested with a new deb package compilation. Works fine. Thanks for your rapidity ^^