Fix crash on invalid curve config

[Smeat]

If one provides an invalid curve configuration with neither linear nor function fan2go crashes with a segmentation fault.

Example config snippets (based on the example config):

curves:
  - id: cpu_curve
    # typo
    linearr:
      sensor: cpu_package
      steps:
        - 40: 0
        - 50: 50
        - 80: 255

or

curves:
  - id: cpu_curve

Crash:

INFO  Using configuration file at: /tmp/fan2go/fan2go.yaml
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x7ae3c2]

No clue why TestValidateCurveSubConfigSensorIdIsMissing doesn't catch this error.
Sorry I am not sure on how to add a test with an actual config file.

[markusressel]

Thx!
This is due to the order of the validation, sensors are validated first.
It should be possible to create a test for this scenario, but I guess this is fine for now.