code: def _next_ifd(self, ifd):
          
          entries = self.s2n(ifd, 2)
          next_ifd = self.s2n(ifd + 2 + 12 * entries, 4)
          if next_ifd == ifd:
              return 0
          else:
              return next_ifd
docs: Return the pointer to next IFD.