Just when you thought it was safe to walk into a flower shop again, “Little Shop of Horrors,” with music by Alan Menken and book and lyrics by Howard Ashman, has returned — this time to the stage of the Mercury Theater, 3745 N. Southport Ave., in Lakeview, and running until April 28.While there have been many frightening fictional monsters like Godzilla, Dracula, King Kong and the Phantom of the Opera, none …