Check time string format before parsing
Reduce more duplicated code
By involving asn1_time_to_tm, we can now get information we mostly need
to print a time string.

This follows what was discussed at
https://github.com/openssl/openssl/pull/4001#discussion_r129092251Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4039)