thread: fix scheduler's handling of async callbacks

If the callback takes too long to run, then a second copy was
scheduled --- which likely schedules a third copy, and so on. This
problem could lead DrRacket to get stuck in a GC cycle, for example.

racket/src/thread/schedule.rkt

@@ -221,21 +221,25 @@
     [(null? callbacks) (swap-in-engine e t leftover-ticks)]
     [else
      (define done? #f)
-     (let loop ([e e])
+     (let loop ([e e] [callbacks callbacks])
        (end-implicit-atomic-mode)
        (e
         TICKS
+        (if (pair? callbacks)
+            ;; run callbacks as a "prefix" callbacks
             (lambda ()
               (run-callbacks callbacks)
               (set! done? #t)
               (engine-block))
+            ;; still running callbacks, so no new prefix
+            void)
         (lambda (e result remaining)
           (start-implicit-atomic-mode)
           (unless e
             (internal-error "thread ended while it should run callbacks atomically"))
           (if done?
               (swap-in-engine e t leftover-ticks)
-              (loop e)))))]))
+              (loop e null)))))]))
 
 ;; Run foreign "async-apply" callbacks, now that we're in some thread
 (define (run-callbacks callbacks)