x509/x509_set.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: 786dd2c2 ("Add support for custom signature parameters")
Signed-off-by: Jiasheng Jiang <jiasheng@purdue.edu>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23956)