Update sm2_crypt.c

asn1_encode : x, y  =>    0 | x，0 | y
(because of DER encoding rules when x and y have high bit set)

CLA: Trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6694)