Add check for public key presence on sm2 signing

Add check for public key presence on sm2 signing

SM2 requires that the public EC_POINT be present in a key when signing.
If its not there we crash on a NULL pointer.  Add a check to ensure that
its present, and raise an error if its not

Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23887)