Nantucket’s own Meghan Trainor stopped by “The Tonight Show Starring Jimmy Fallon’’ Monday — and she managed to remain upright during the entire performance! Decked out in a giraffe onesie, the “All About the Bass’’ singer joked with Fallon about falling onstage during her last visit to the show. “I can’t be the only person that’s fallen on this stage,’’ she said. You’ll recall that after she tipped over in May, Fallon gamely laid down next to her. (Fallon, of course, is hardly unfamiliar with falling down: He injured his hand in a fall at Harvard last October.) On Monday’s show, Fallon donned a dinosaur onesie that Trainor gave him and he replayed Trainor’s infamous fall.