Fix a double free issue when signing SM2 cert

Fix a double free issue when signing SM2 cert

If the SM2 ID value has not been passed correctly when signing an SM2
certificate/certificate request, a double free occurs. For instance:

  openssl req -x509 ... -sm2-id 1234567812345678

The '-sm2-id' should not be used in this scenario, while the '-sigopt' is
the correct one to use. Documentation has also been updated to make the
options more clear.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9958)