Add SM2 private key range validation

Add SM2 private key range validation

According to the relevant standards, the valid range for SM2 private
keys is [1, n-1), where n is the order of the curve generator.

For this reason we cannot reuse the EC validation function as it is, and
we introduce a new internal function `sm2_key_private_check()`.

Partially fixes https://github.com/openssl/openssl/issues/8435

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13359)