prop:evt: support chaperone when extracting field

Thanks to Sam for noticing this problem.

pkgs/racket-pkgs/racket-test/tests/racket/chaperone.rktl

@@ -1258,6 +1258,18 @@
                             (set! checked? #t)
                             v)))
   (test #t values checked?))
+(let ()
+  (struct e (orig)
+    #:property prop:evt 0)
+  (define an-e (e always-evt))
+  (define checked? #f)
+  (test #t evt? an-e)
+  (sync (chaperone-struct an-e
+                          e-orig
+                          (lambda (self v)
+                            (set! checked? #t)
+                            v)))
+  (test #t values checked?))
 
 ;; ----------------------------------------
 ;; channel chaperones

racket/src/racket/src/struct.c

@@ -1521,7 +1521,7 @@ static int evt_struct_is_ready(Scheme_Object *o, Scheme_Schedule_Info *sinfo)
   }
 
   if (SCHEME_INTP(v))
-    v = ((Scheme_Structure *)o)->slots[SCHEME_INT_VAL(v)];
+    v = scheme_struct_ref(o, SCHEME_INT_VAL(v));
 
   if (scheme_is_evt(v)) {
     scheme_set_sync_target(sinfo, v, NULL, NULL, 0, 1, NULL);