The China arm of the world’s largest retailer Walmart Inc has received a warning from police in the southern tech hub of Shenzhen for breaches of cybersecurity laws, as the country tightens its grip over how data is handled by businesses.The public security authorities in Shenzhen found 19 cybersecurity loopholes in November in the online network of Walmart’s China operation, which could be susceptible to exploitation, and the company did not subsequently fix the issues in a timely manner,…