ui/vnc: don't raise error formatting socket address for non-inet

ui/vnc: don't raise error formatting socket address for non-inet

The SASL library requires the connection's local & remote IP address to
be passed in, since some mechanism may use this information. Currently
QEMU raises an error for non-inet sockets, but it is valid to pass NULL
to the SASL library. Doing so makes SASL work on UNIX sockets.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>