Return the pointer to next IFD.

def _next_ifd(self, ifd):
        """Return the pointer to next 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