ossl_cmp_certreq_new(): Fix POPO key mismatch in case newPkey is just public key

Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13409)

crypto/cmp/cmp_msg.c

@@ -334,7 +334,12 @@ OSSL_CMP_MSG *ossl_cmp_certreq_new(OSSL_CMP_CTX *ctx, int type,
     if (type != OSSL_CMP_PKIBODY_P10CR) {
         EVP_PKEY *privkey = OSSL_CMP_CTX_get0_newPkey(ctx, 1);
 
-        if (privkey == NULL)
+        /*
+         * privkey is NULL in case ctx->newPkey does not include a private key.
+         * We then may try to use ctx->pkey as fallback/default, but only
+         * if ctx-> newPkey does not include a (non-matching) public key:
+         */
+        if (privkey == NULL && OSSL_CMP_CTX_get0_newPkey(ctx, 0) == NULL)
             privkey = ctx->pkey; /* default is independent of ctx->oldCert */
         if (ctx->popoMethod == OSSL_CRMF_POPO_SIGNATURE && privkey == NULL) {
             ERR_raise(ERR_LIB_CMP, CMP_R_MISSING_PRIVATE_KEY);