Intercept a NULL s early in ASN1_TIME_set_string_internal()

Intercept a NULL s early in ASN1_TIME_set_string_internal()

If s is NULL, the only thing the tm_to_*() functions do is a check that
a GeneralizedTime has a four digit year (between 0000 and 9999) and a
UTCTime has a year between 1950 and 2050. These checks are already done
in ASN1_TIME_parse() itself: the century is 100 times a two-digit value
(or 19 in the UTCTime case) plus another two-digit value.

ok beck