Fix ASN1_TIME_diff() with NULL times

Fix ASN1_TIME_diff() with NULL times

The ASN1_TIME_diff() API accepts NULL ASN1_TIMEs and interprets them
as "now". This is used in sysutils/monit, as found by semarie with a
crash after update. Implement this behavior by porting a version of
ASN1_TIME_to_tm() to LibreSSL and using it in ASN1_TIME_diff().

Tested by semarie

ok beck jsing semarie