fix sync on place with no writers

A `sync` on a place whose initial channel is known to have no writers
could trigger a place result instead of blocking on the place.

pkgs/racket-pkgs/racket-test/tests/racket/place-no-writer.rkt

@@ -0,0 +1,20 @@
+#lang racket/base
+(require racket/place)
+
+;; Check that syncing on a place (as opposed to place channel)
+;; known to have no writers will fail to sync (as opposed to
+;; returning the place itself, for example, which was the bug
+;; that triggered this test).
+
+(define (main)
+  (for ([i 10])
+    (displayln i)
+    (define p (place pch (void)))
+    (place-wait p)
+    (collect-garbage)
+    (define v (sync/timeout 0 p))
+    (when v
+      (error "failed: " v))))
+
+(module+ main (main))
+(module+ test (main))

racket/src/racket/src/place.c

@@ -3487,10 +3487,7 @@ static int place_channel_ready(Scheme_Object *so, Scheme_Schedule_Info *sinfo) {
 
     return 1;
   }
-
+  
-  if (no_writers)
-    return 1; /* wake up to discover that we can give up */
-
   return 0;
 }