vnc-auth-sasl: Plug memleak in vnc_socket_ip_addr_string

'addr' is forgot to free in vnc_socket_ip_addr_string error path. Fix that.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-Id: <20200831134315.1221-11-pannengyuan@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>