It was a dark and stormy night, and some Boston police officers making a hamburger run Tuesday at the South End restaurant The Gallows found their wagon stuck in a snowbank.When what to their wondering eyes did appear, not a miniature sleigh and eight tiny reindeer, but Queen Elsa — taller and more muscular somehow than the character from the Disney movie “Frozen” — who gave them a push as one of the cops tried to steer.