Tragic Barnard College freshman Tessa Majors was allegedly in Morningside Park to buy marijuana when she was fatally stabbed by a group of teenage robbers, the head of the NYPD sergeants’ union claimed on Sunday. “What I am understanding is that [Majors] was in the park to buy marijuana,” Sergeants Benevolent Association president Ed Mullins told radio host John Catsimatidis on his AM 970 show, The Cats Roundtable. Police sources confirmed to The Post that they are investigating that angle, based on claims from a college friend of Majors’. A friend of the victim claimed to detectives that the 18-year-old...