block/win32: fix 'ret not initialized' warning

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170516074256.24731-1-kraxel@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>