fix invalid use of incomplete type X509_STORE_CTX

CLA: trivial
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

doc/ssl/SSL_CTX_set_verify.pod

@@ -224,7 +224,7 @@ L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>).
      */
     if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT))
     {
-      X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
+      X509_NAME_oneline(X509_get_issuer_name(err_cert), buf, 256);
       printf("issuer= %s\n", buf);
     }