panic when natTraversal=true and NATBlocked

[shyer]

deploy by helm:
https://github.com/openyurtio/raven/blob/main/docs/raven-agent-tutorial.md

raven/charts/raven-agent/values.yaml

Line 69 in a27a77e

natTraversal: false

stun return host nil when NATBlocked
https://github.com/ccding/go-stun/blob/d32c135f5e509d278e95ba9945849903f966a91f/stun/discover.go#L76

then panic nil

raven/pkg/utils/stun.go

Line 67 in a27a77e

PublicPort = int(host.Port())

[alokkumardalei-wq]

@shyer ,just raised a pr on this please a have a look when you get time.
Happy to get any kind of feedback on this :).