If SetSessionTicketKeys was called on a fresh tls.Config, the configured
keys would be overridden with a random key by serverInit.

Fixes #15421.

Change-Id: I5d6cc81fc3e5de4dfa15eb614d102fb886150d1b
Reviewed-on: https://go-review.googlesource.com/27317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>