Deal with the consequences of constifying getters

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
diff --git a/apps/x509.c b/apps/x509.c
index 0753817..4191a34 100644
--- a/apps/x509.c
+++ b/apps/x509.c
@@ -684,14 +684,14 @@
                 BIO_printf(out, "Modulus=");
 #ifndef OPENSSL_NO_RSA
                 if (EVP_PKEY_id(pkey) == EVP_PKEY_RSA) {
-                    BIGNUM *n;
+                    const BIGNUM *n;
                     RSA_get0_key(EVP_PKEY_get0_RSA(pkey), &n, NULL, NULL);
                     BN_print(out, n);
                 } else
 #endif
 #ifndef OPENSSL_NO_DSA
                 if (EVP_PKEY_id(pkey) == EVP_PKEY_DSA) {
-                    BIGNUM *dsapub = NULL;
+                    const BIGNUM *dsapub = NULL;
                     DSA_get0_key(EVP_PKEY_get0_DSA(pkey), &dsapub, NULL);
                     BN_print(out, dsapub);
                 } else