isisd: fix isis_circuit_af_set() on fresh circuit

isisd: fix isis_circuit_af_set() on fresh circuit

A newly-created circuit will be in enabled state but have neither IPv4
nor IPv6 configured.  The logic in isis_circuit_af_set assumed that
"enabled" is equivalent to "ip || ipv6".

This is the only place where this distinction is currently relevant, as
the CLI won't allow enabling an interface without enabling either IPv4
or IPv6;  and it will also disable a circuit when both are deconfigured.

Reported-by: Martin Winter <mwinter@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>