Fixed bug in audio conference disconnection

[sauwming]

To fix #4196.

Related to #3928, the loops to disconnect multiple connections from audio ports (any->sink, or source->any) are incorrect due to the potentially changing listener array and its count.

        for (i=0; i<src_port->listener_cnt; ++i) {
            ...
            // But the listener count will decrease here, and its array will shift.
            op_disconnect_ports(conf, &op_prm);
        }