Fix printing of DH Parameters

The -text argument to dhparam is broken, because the DHparams_print()
function always returns an error. The problem is that always expects a
public or private key to be present, even though that is never the case
with parameters.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/dh/dh_ameth.c

@@ -280,7 +280,8 @@ static int do_dh_print(BIO *bp, const DH *x, int indent, int ptype)
     else
         pub_key = NULL;
 
-    if (priv_key == NULL && pub_key == NULL) {
+    if (x->p == NULL || (ptype == 2 && priv_key == NULL)
+            || (ptype > 0 && pub_key == NULL)) {
         reason = ERR_R_PASSED_NULL_PARAMETER;
         goto err;
     }