Prevent Fiber#initialize to be called twice; fix #3705

mrbgems/mruby-fiber/src/fiber.c

@@ -74,6 +74,9 @@ fiber_init(mrb_state *mrb, mrb_value self)
 
   mrb_get_args(mrb, "&", &blk);
 
+  if (f->cxt) {
+    mrb_raise(mrb, E_RUNTIME_ERROR, "cannot initialize twice");
+  }
   if (mrb_nil_p(blk)) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "tried to create Fiber object without a block");
   }