time64: fix signed overflow for extreme years in gmtime64_r()

IS_LEAP() computed `tm_year + 1900` in int arithmetic, which overflows
for dates around year 2^31 (hit via check_tm() when writing dates).

Cast to Year (64-bit) in IS_LEAP(). Also make gmtime64_r() return NULL
with EOVERFLOW when the calendar year (tm_year + 1900) does not fit in
an int; the previous check compared tm_year with itself and could never
fail. This avoids strftime() wrapping the year in the output, and
callers fall back to their existing default date string.

Credit to OSSFuzz

0 files changed, 0 insertions, 0 deletions