Private broadcast connections use either Tor or I2P, which require a
proxy intrinsically or IPv4 or IPv6 which must use a proxy in the
context of private broadcast to avoid leaking the originator's IP
address.
Add a safety check to guard against future mistakes.
Co-authored-by: Andrew Toth <andrewstoth@gmail.com>
Github-Pull: #35410
Rebased-From: d01b461f71