qemu-nbd: Fix a memleak in qemu_nbd_client_list()

When the qio_channel_socket_connect_sync() fails
we should goto 'out' label to free the 'sioc' instead of return.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Message-Id: <20201130123651.17543-1-alex.chen@huawei.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Eric Blake <eblake@redhat.com>