SHANGHAI, Aug 24 (Reuters) - Chinese food delivery giant Meituan (3690.HK) on Thursday posted a bigger-than-expected 33.4% rise in second-quarter revenue, defying a slowing Chinese economy, though executives warned of headwinds for its core business in the quarter ahead. Meituan, which offers an app that provides services such as bike-sharing, ticket-booking and maps, reported April-June revenue of 67.9 billion yuan ($9.33 billion), up from 50.9 billion a year earlier. That beat the 66.7 billion expected by 15 analysts in estimates compiled by Refinitiv Eikon. It posted a net profit of 4.7 billion yuan versus a loss of 1.1 billion yuan...