Box: Mark boxes when a class/module is originally defined in it.

When a class/module defined by extension libraries in a box, checking
types of instances of the class needs to access its data type (rb_data_type_t).
So if a class still exists (not GCed), the box must exist too (to be marked).

Author: tagomoris

gc.c

@@ -3154,12 +3154,18 @@ rb_gc_mark_children(void *objspace, VALUE obj)
         foreach_args.objspace = objspace;
         foreach_args.obj = obj;
         rb_class_classext_foreach(obj, gc_mark_classext_module, (void *)&foreach_args);
+        if (BOX_USER_P(RCLASS_PRIME_BOX(obj))) {
+            gc_mark_internal(RCLASS_PRIME_BOX(obj)->box_object);
+        }
         break;
 
       case T_ICLASS:
         foreach_args.objspace = objspace;
         foreach_args.obj = obj;
         rb_class_classext_foreach(obj, gc_mark_classext_iclass, (void *)&foreach_args);
+        if (BOX_USER_P(RCLASS_PRIME_BOX(obj))) {
+            gc_mark_internal(RCLASS_PRIME_BOX(obj)->box_object);
+        }
         break;
 
       case T_ARRAY: