Fix counting/ checking for leap years in mktime_utc()
When HAVE_TIMEGM is undefined mktime_utc() might output one day shift (+86400s)
for (years <= 1967 or years >= 2100) && month >= 3
{ .tm_mday = 1 .tm_mon = 2 .tm_year = 67 }
mktime_utc() = -
89436590 // Thu Mar 2 00:00:00 UTC 1967
gmtime() = -
89596800 // Wed Mar 1 00:00:00 UTC 1967
{ .tm_mday = 1 .tm_mon = 2 .tm_year = 200 }
mktime_utc() =
4107628800 // Tue Mar 2 00:00:00 UTC 2100
gmtime() =
4107542400 // Mon Mar 1 00:00:00 UTC 2100
Change-Id: I1a544762fa5178c8798496d7dc30a2e767919149
Reviewed-on: https://code.wireshark.org/review/1287
Reviewed-by: Anders Broman <a.broman58@gmail.com>