Fix #71894: AddressSanitizer: global-buffer-overflow in zif_cal_from_jd

Fix #71894: AddressSanitizer: global-buffer-overflow in zif_cal_from_jd

Julian days < 347998 denote invalid Jewish calendar dates, so
cal_from_jd($jd, CAL_JEWISH) and jdmonthname($jd, CAL_MONTH_JEWISH) should
actually fail. For BC we don't yet let them though, but we fix the OOB read
that happens in this case, and we also adjust cal_from_jd()'s return value
to have empty strings for "abbrevdayname" and "dayname" instead of "Sun"/
"Sunday" and NULL for "dow" instead of 0, which doesn't make any sense.