fix peek implementation in R6RS input ports

The peek operation must not block, otherwise the port behaves
badly for `sync`.

Based on a bug report from Brett Stahlman.

pkgs/r6rs-pkgs/r6rs-lib/r6rs/private/ports.rkt

@@ -164,7 +164,7 @@
             n)))
     (lambda (bytes skip evt)
       (check-disconnect)
-      (let ([n (peek-bytes-avail! bytes skip evt port)])
+      (let ([n (peek-bytes-avail!* bytes skip evt port)])
         (if (eq? n 0)
             (wrap-evt port (lambda (v) 0))
             n)))

pkgs/r6rs-pkgs/r6rs-test/tests/r6rs/io/sync.rkt

@@ -0,0 +1,6 @@
+#lang racket
+(require rnrs/io/ports-6)
+
+;; Make sure that an R6RS port plays ok with sync,
+;; particularly when no input is available.
+(void (sync/timeout 0 (standard-input-port)))