clear `stack_end`.

* cont.c (cont_save_thread): clear only `stack_end`. Clearing tells
  GC mark function to ignore this macine stack (not allocated yet).
  `stack_start` will be used by machine stack store/restore phase
  (on FIBER_USE_NATIVE == 0), so that only `stack_end` is cleared.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59829 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

cont.c

@@ -485,16 +485,14 @@ cont_save_thread(rb_context_t *cont, rb_thread_t *th)
     /* save thread context */
     sth->ec = th->ec;
 
-#if FIBER_USE_NATIVE
-    /* saved_thread->machine.stack_(start|end) should be NULL */
+    /* saved_thread->machine.stack_end should be NULL */
     /* because it may happen GC afterward */
-    sth->ec.machine.stack_start = NULL;
     sth->ec.machine.stack_end = NULL;
+
 #ifdef __ia64
     sth->ec.machine.register_stack_start = NULL;
     sth->ec.machine.register_stack_end = NULL;
 #endif
-#endif
 }
 
 static void