Ma Xingrui, a member of China's elite politburo who had risen rapidly through the ranks after working as a rocket scientist, is under investigation over suspected "serious violation of law and discipline".China is stepping up its years-long fight to root out corruption with the purge of sitting members of the politburo, the ruling Communist Party's decision-making body, with Ma the third member targeted in the past six months.Ma, who also serves as the deputy leader of the central rural work leading group, is undergoing a disciplinary review and supervisory investigation, the Central Commission for Discipline Inspection said in a notice…