crypto: hisilicon - Fix using plain integer as NULL pointer

This patch fix sparse warning:
zip_crypto.c:425:26: warning: Using plain integer as NULL pointer

Replaces assignment of 0 to pointer with NULL assignment.

Signed-off-by: Shukun Tan <tanshukun1@huawei.com>
Signed-off-by: Zhou Wang <wangzhou1@hisilicon.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>