panic in prometheus input due to nil about cpthook

[skorpy2009]

This probably comes from https://github.com/fleaz/CptHook/blob/master/input/prometheus.go#L154
The alert is:

POST /prometheus HTTP/1.1
Host: 127.0.0.1:8086
User-Agent: Alertmanager/0.15.2
Content-Length: 1012
Content-Type: application/json

{"receiver":"irc","status":"firing","alerts":[{"status":"firing","labels":{"alertname":"BGP6TransitSessionDown","severity":"page"},"annotations":{"description":"\u003e=2 BGP IPv6 Transit session are down for more than 1 minute","summary":"Instance : BGP6  down","value":""},"startsAt":"2018-10-21T12:50:32.490632004+02:00","endsAt":"2018-10-22T17:13:32.490632004+02:00","generatorURL":"https://prometheus.ffm.freifunk.net/graph?g0.expr=count%28%28avg_over_time%28bird_bgp6_session_up%7Bname%3D~%22er.%2Aof_aixit_net%7CHurricane_Electric_KLEYREX%22%7D%5B1m%5D%29+%3D%3D+0%29%29+%3E+1\u0026g0.tab=1"}],"groupLabels":{"alertname":"BGP6TransitSessionDown"},"commonLabels":{"alertname":"BGP6TransitSessionDown","severity":"page"},"commonAnnotations":{"description":"\u003e=2 BGP IPv6 Transit session are down for more than 1 minute","summary":"Instance : BGP6  down","value":""},"externalURL":"https://prometheus.ffm.freifunk.net/alertmanager","version":"4","groupKey":"{}/{}:{alertname=\"BGP6TransitSessionDown\"}"}