fix `ffi/unsafe/nsalloc'

Using `call-as-atomic' isn't right, because that allows an escape
via `call-as-nonatomic'. Assuming that `call-as-nonatomic' isn't
used, it seems like `call-as-atomic' should be ok, anyway, but
somehow its leads to unbalanced `end-atomic' calls.

collects/ffi/unsafe/nsalloc.rkt

@@ -18,11 +18,13 @@
 (define (call-with-autorelease thunk)
   (unless NSAutoreleasePool
     (error 'NSAutoreleasePool "not available"))
-  (call-as-atomic
+  (dynamic-wind
+   start-atomic
    (lambda ()
      (let ([pool (tell (tell NSAutoreleasePool alloc) init)])
        (dynamic-wind
            void
            thunk
            (lambda ()
-             (tellv pool release)))))))
+             (tellv pool release)))))
+   end-atomic))

src/racket/src/thread.c

@@ -4984,7 +4984,11 @@ void scheme_start_atomic_no_break(void)
 
 void scheme_end_atomic_no_swap(void)
 {
-  --do_atomic;
+  int v = --do_atomic;
+  if (v < 0) {
+    scheme_log_abort("unbalanced end-atomic");
+    abort();
+  }
 }
 
 void scheme_start_in_scheduler(void)
@@ -4995,8 +4999,12 @@ void scheme_start_in_scheduler(void)
       
 void scheme_end_in_scheduler(void)
 {
-  --do_atomic;
+  int v = --do_atomic;
   --scheme_no_stack_overflow;
+  if (v < 0) {
+    scheme_log_abort("unbalanced end-atomic");
+    abort();
+  }
 }
 
 void scheme_end_atomic(void)