Fix get_delay() code to avoid TIME_MAX overflow, not INT_MAX.

  o Minor bugfixes (download timing):

    - When determining when to download a directory object, handle times

      after 2038 if the operating system supports that.  (Someday this will be

      important!)  Fixes bug 20587; bugfix on 0.2.8.1-alpha.

   * non-negative allows us to safely do the wrapping check below. */

  tor_assert(delay >= 0);

  /* Avoid now+delay overflowing INT_MAX, by comparing with a subtraction

  /* Avoid now+delay overflowing TIME_MAX, by comparing with a subtraction

   * that won't overflow (since delay is non-negative). */

  if (delay < INT_MAX && now <= INT_MAX - delay) {

  if (delay < INT_MAX && now <= TIME_MAX - delay) {

    dls->next_attempt_at = now+delay;

  } else {

    dls->next_attempt_at = TIME_MAX;