racket/port: fix busy-wait bug in make-limited-input-port

As reported by Marc Burns on the mailing list

pkgs/racket-pkgs/racket-test/tests/racket/portlib.rktl

@@ -649,6 +649,18 @@
       (test 2 peek-bytes-avail!* b 0 #f s)
       (test 1 peek-bytes-avail!* b 1 #f s)
       (test 2 read-bytes-avail!* b s))))
+
+;; Make sure that blocking on a limited input port doesn't
+;; block in the case of a peek after available bytes:
+(let ()
+  (define-values (i o) (make-pipe))
+  (write-char #\a o)
+  (write-char #\a o)
+  
+  (thread (lambda ()
+            (sync (system-idle-evt))
+            (write-char #\b o)))
+  (test 1 peek-bytes-avail! (make-bytes 1) 2 #f (make-limited-input-port i 10)))
 	     
 ;; ----------------------------------------
 ;; Conversion wrappers

racket/collects/racket/port.rkt

@@ -1035,7 +1035,11 @@
                 (if (eq? n 0)
                     (if (and progress-evt (sync/timeout 0 progress-evt))
                         #f
-                        (wrap-evt port (lambda (x) 0)))
+                        (wrap-evt (if (zero? skip)
+                                      port
+                                      (choice-evt (or progress-evt never-evt)
+                                                  (peek-bytes-evt 1 skip progress-evt port)))
+                                  (lambda (x) 0)))
                     n)))))
       (define (try-again)
         (wrap-evt