place: fix finalizaiton of place channels stuck in a GC'ed channel

Lack of ordering on finalization requires some care for explicit
clean-up actions in a finalizer.

pkgs/racket-pkgs/racket-test/tests/racket/place-in-channel-fnl.rkt

@@ -0,0 +1,15 @@
+#lang racket/base
+(require racket/place)
+
+;; Check finalization of place channels that are
+;; abandoned in unreferenced place channels. 
+(let loop ([n 0])
+  (unless (= n 50000)
+    (define-values (i o) (place-channel))
+    (define-values (i2 o2) (place-channel))
+    (place-channel-put i o2)
+    (place-channel-put o i2)
+    (loop (add1 n))))
+'ok
+
+

racket/src/racket/src/place.c

@@ -2982,6 +2982,11 @@ Scheme_Place_Async_Channel *place_async_channel_create() {
 
 static void async_channel_refcount(Scheme_Place_Async_Channel *ch, int for_send, int delta)
 {
+  if (!ch->lock) {
+    /* can happen via finalization, where the channel is already finalized
+       m(due to the lack of ordering on finalization) */
+    return;
+  }
   mzrt_mutex_lock(ch->lock);
   if (for_send)
     ch->wr_ref += delta;