thread: add missing end-atomic

racket/src/thread/atomic.rkt

@@ -41,6 +41,7 @@
      (set! end-atomic-callback #f)
      (current-atomic n)
      (cb)]
+    [(negative? n) (internal-error "not in atomic mode to end")]
     [else
      (current-atomic n)]))
 

racket/src/thread/sync.rkt

@@ -125,6 +125,7 @@
                   (end-atomic)
                   (loop #f #f)]
                  [else
+                  (end-atomic)
                   ;; Return result in a thunk:
                   (lambda () #f)])]
               [(and (all-asynchronous? s)