Robert De Niro is on the mend. The “Goodfellas” star, 82, was photographed with his right arm in a sling while attending a Broadway performance of “Oedipus” at Studio 54 in New York City on Wednesday. De Niro stepped backstage after the show, where he greeted cast members and showed support for the production. A rep confirmed to People that the “Raging Bull” star recently underwent a procedure and will be wearing the sling for at least a few more days. The Post has reached out to De Niro’s rep