bc: fix sync when a guard-evt turns into choice-evt late

During the time that the procedure in a guard-evt is called by `sync`,
it's possible for the `sync` evt choice to become determined. If the
guard-evt procedure then returns a choice-evt, the BC implementation
of `sync` could lose track of the selected evt; the selection is
represented by an index, and choice-evt splicing can shift indices.

Possibly related to #3004

pkgs/racket-test-core/tests/racket/sync.rktl

@@ -1651,6 +1651,26 @@
     (sync s))
   (thread-wait t))
 
+;; ----------------------------------------
+;; regression test to check that when a choice evt replaces a single
+;; event, an already-chosen event (perhaps represented as an index) is
+;; not misinterpreted later
+
+(for ([i (in-range 10)])
+  (define ch (make-channel))
+  (define v
+    (sync (guard-evt
+           (lambda ()
+             (thread (lambda () (channel-put ch 0)))
+             (sync (system-idle-evt))
+             (choice-evt
+              (wrap-evt always-evt (lambda (v) 1))
+              (wrap-evt always-evt (lambda (v) 2))
+              (wrap-evt always-evt (lambda (v) 3)))))
+          ch))
+  (unless (memq v '(0 1 2 3))
+    (error "bad sync result" v)))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/src/bc/src/thread.c

@@ -6560,7 +6560,10 @@ static void set_sync_target(Syncing *syncing, int i, Scheme_Object *target,
       /* Inline the set (in place) */
       Scheme_Object **argv;
       Evt **ws;
-       
+
+      if (syncing->result > i+1)
+        syncing->result += wts->argc-1;
+
       argv = (Scheme_Object **)splice_ptr_array((void **)evt_set->argv, 
 						evt_set->argc,
 						(void **)wts->argv,