signature/sm2_sig.c: Add the check for the EVP_MD_CTX_get_size()

Add the check for the return value of EVP_MD_CTX_get_size() to avoid invalid negative numbers.

Fixes: d0b79f86 ("Add SM2 signature algorithm to default provider")
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23959)