fix problem with multiple captures of the same continuation

--- a bug introduced when enabling full continuations
     to escape past continuation barriers in v5.0.1
 Closes PR 11409

collects/tests/racket/basic.rktl

@@ -1836,6 +1836,19 @@
             ((car x) (lambda () x))
             (pair? (x)))))
 
+;; Check shared escape continuation of nested call/cc:
+(let ([ch (make-channel)])
+ (thread
+  (lambda ()
+    (channel-put 
+     ch
+     (call/cc
+      (lambda (escape)
+        (call/cc
+         (lambda (escape1)
+           (escape1 3))))))))
+ (sync ch))
+
 (arity-test call/cc 1 2)
 (arity-test call/ec 1 1)
 (err/rt-test (call/cc 4))

src/racket/src/fun.c

@@ -6181,7 +6181,7 @@ internal_call_cc (int argc, Scheme_Object *argv[])
       && (sub_cont->prompt_tag == prompt_tag)
       && (sub_cont->barrier_prompt == effective_barrier_prompt)
       && (((Scheme_Escaping_Cont *)sub_cont->escape_cont)->myerr == p->error_buf)) {
-    /* Whether sub_cont turns out to be the same continuaiton, we can use
+    /* Whether sub_cont turns out to be the same continuation, we can use
        its escape continuation, because jumping to the escape continuation
        triggers the same C-level clean-up actions, same `dynamic-wind's, and
        crosses the same continuation barriers. */
@@ -6230,10 +6230,10 @@ internal_call_cc (int argc, Scheme_Object *argv[])
       cont = MALLOC_ONE_TAGGED(Scheme_Cont);
       cont->so.type = scheme_cont_type;
       cont->buf.cont = sub_cont;
-      sub_cont = sub_cont->buf.cont;
-
       cont->escape_cont = sub_cont->escape_cont;
 
+      sub_cont = sub_cont->buf.cont;
+
       /* This mark stack won't be restored, but it may be
 	 used by `continuation-marks'. */
       cont->ss.cont_mark_stack = MZ_CONT_MARK_STACK;