req: Do not warn about using stdin when generating new request

Fixes #16773

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16774)

apps/req.c

@@ -492,7 +492,7 @@ int req_main(int argc, char **argv)
     if (infile == NULL) {
         if (gen_x509)
             newreq = 1;
-        else
+        else if (!newreq)
             BIO_printf(bio_err,
                        "Warning: Will read cert request from stdin since no -in option is given\n");
     }