A murder suspect was nabbed when he was stopped for smoking in the subway by NYPD cops who then quickly discovered he was being sought for a 2020 Queens slaying, police said Monday. On Friday, transit cops in Brooklyn stopped Luis Hernandez, 36, for smoking. He would have been released with just a summons but when officers ran his name they learned he was wanted for murder. He was taken to Queens, where detectives showed him video and photos of him at the murder scene on 31st St. near 36th Ave. in Astoria, police said. Police say Hernandez and three or four others were involved in the Feb. 9, …