generator.c: trigger write barriers in cState_init_copy

Reported-By: Yuhang Wu <yuhang@depthfirst.com>

Author: byroot

ext/json/ext/generator/generator.c

@@ -1367,12 +1367,14 @@ static VALUE cState_init_copy(VALUE obj, VALUE orig)
     if (!objState) rb_raise(rb_eArgError, "unallocated JSON::State");
 
     MEMCPY(objState, origState, JSON_Generator_State, 1);
-    objState->indent = origState->indent;
-    objState->space = origState->space;
-    objState->space_before = origState->space_before;
-    objState->object_nl = origState->object_nl;
-    objState->array_nl = origState->array_nl;
-    objState->as_json = origState->as_json;
+
+    RB_OBJ_WRITTEN(obj, Qundef, objState->indent);
+    RB_OBJ_WRITTEN(obj, Qundef, objState->space);
+    RB_OBJ_WRITTEN(obj, Qundef, objState->space_before);
+    RB_OBJ_WRITTEN(obj, Qundef, objState->object_nl);
+    RB_OBJ_WRITTEN(obj, Qundef, objState->array_nl);
+    RB_OBJ_WRITTEN(obj, Qundef, objState->as_json);
+
     return obj;
 }