The controlhttp dialer with a ControlDialPlan IPv6 entry was hitting a
case where the dnscache Resolver was returning an netip.Addr zero
value, where it should've been returning the IPv6 address.
We then tried to dial "invalid IP:80", which would immediately fail,
at least locally.
Mostly this was causing spammy logs when debugging other stuff.
Updates tailscale/corp#32534
Change-Id: If8b9a20f10c1a6aa8a662c324151d987fe9bd2f8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>