[php-src] PR #23971: Fix cal_days_in_month() returning a negative day count at the calendar's boundary year

Pull Request: https://github.com/php/php-src/pull/23971 Author: lacatoire cal_days_in_month() computes a month's length by comparing its start Julian Day Number to the next month's. For December, that means looking up January 1st of year + 1. For the last representable year of the Gregorian/Julian calendars (2147478847, i.e. INT32_MAX - 4800), year + 1 is out of range and the internal lookup returns 0 instead of a real date. Only CAL_FRENCH had a fallback for this end-of-range case; every other calendar silently subtracted against that 0 and returned a large negative day count instead of raising. ```php var_dump(cal_days_in_month(CAL_GREGORIAN, 12, 2147478847)); // int(-138352149257414216) before this fix // ValueError: Invalid date, after ``` Fix: treat a still-zero sdn_next as an invalid date for every calendar, not just CAL_FRENCH.