cifsd: add the check if nvec is zero

Dan Carpenter pointed out that memory can be corrupted when nvec is zero.
This patch add the check to prevent it.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>