The GitHub CodeQL scanner flagged the localapi's cert domain usage as a problem
because user input in the URL made it to disk stat checks.
The domain is validated against the ipnstate.Status later, and only
authenticated root/configured users can hit this, but add some
paranoia anyway.
Change-Id: I373ef23832f1d8b3a27208bc811b6588ae5a1ddd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
