asn1/a_time.c: make handling of 'fractional point' formally correct.

Even though tm->length >= 15 && v[14] == '.' works in practice,
[because "YYYYMMDDHHMMSS." would be rejected as invalid by
asn1_time_to_tm,] formal correctness with respect to buffer
overstep in few lines vicinity improves readability.

[Also fold one if condition and improve expression readability.]

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4058)