Society photographer Billy Farrell has shot his share of rock stars, but he unwittingly became one Saturday night at his surprise birthday bash. He found cheering friends and family, musical equipment and members of his old band Kallit Molly ready to reunite when he wandered into Chrystie Street speakeasy Fig 19. After a quick rehearsal next door at art space Envoy Enterprises, Farrell kicked out a set of guitar jams, including an ode to fashion editor André Leon Talley (“People think you’re scary . . . I think you’re smart”).