Erase temporary buffer in EVP_PKEY_get_bn_param()

Function EVP_PKEY_get_bn_param() uses temporary buffer (on stack or
heap allocated) to store serialized bignum, but after deserializing it
into BIGNUM*, the buffer is not erased and may contain sensitive data.

This change makes sure the buffer is erased if it was successfully
filled before. Unfortunately, it does not distinguish between public and
private key components, and will always erase the buffer.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #20639)

(cherry picked from commit 34e4a96)

crypto/evp/p_lib.c

@@ -2178,7 +2178,14 @@ int EVP_PKEY_get_bn_param(const EVP_PKEY *pkey, const char *key_name,
         goto err;
     ret = OSSL_PARAM_get_BN(params, bn);
 err:
-    OPENSSL_free(buf);
+    if (buf != NULL) {
+        if (OSSL_PARAM_modified(params))
+            OPENSSL_clear_free(buf, buf_sz);
+        else
+            OPENSSL_free(buf);
+    } else if (OSSL_PARAM_modified(params)) {
+        OPENSSL_cleanse(buffer, params[0].data_size);
+    }
     return ret;
 }