x/net: possible nil deref in https2 server

[wooffie]

https://cs.opensource.google/go/x/net/+/master:http2/server.go;l=2618-2621

We checking b.conn for nil -> func accepts that this can happened (b.conn == nil)

But after we dereference b.conn that can be nil

[seankhliao]

is there a real case of nil dereference panics?