need to call write_barrier on every fiber context switch

mrbgems/mruby-fiber/src/fiber.c

@@ -280,6 +280,7 @@ fiber_transfer(mrb_state *mrb, mrb_value self)
     mrb->c = c;
     c->status = MRB_FIBER_RUNNING;
     MARK_CONTEXT_MODIFY(c); 
+    mrb_write_barrier(mrb, (struct RBasic*)c->fib); 
     return fiber_result(mrb, a, len);
   }
 
@@ -310,6 +311,7 @@ mrb_fiber_yield(mrb_state *mrb, mrb_int len, const mrb_value *a)
   mrb->c = c->prev;
   c->prev = NULL;
   MARK_CONTEXT_MODIFY(mrb->c);
+  mrb_write_barrier(mrb, (struct RBasic*)c->fib); 
   return fiber_result(mrb, a, len);
 }