gdbusaddress: Validate the noncefile attribute of nonce-tcp addresses

Doing this mostly to fix a compiler warning about tautological
assignments on Android.

See the D-Bus specification:
https://dbus.freedesktop.org/doc/dbus-specification.html#transports-nonce-tcp-sockets

Signed-off-by: Philip Withnall <withnall@endlessm.com>