fix out-of-bounds write in sm2_crypt.c

asn1_encode has two form length octets: short form(1 byte), long form(1+n byte).

CLA: Trivial

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7027)