STAMFORD — Police say a tip led to the discovery of another batch of unmarked heroin similar to the bags found near the bodies of three people who died of overdoses in the city last month. Capt. Richard Conklin said officers in the Narcotics and Organized Crime squad were alerted last week to a man in his late 20s who was expected to make a heroin delivery at the McDonald’s on East Main Street. Conklin said officers conducted surveillance of the parking lot and spotted a man fitting the suspect’s description in a Ford Expedition.