We didn't update the includer field during compaction so it could become
a dangling pointer after compaction. It's only recently that we started
to dereference the field, and we were only comparing the pointer before
then, so the omission only recently started to cause crashes.

By instrumenting object.c:833 with `rp(includer);`, you can see the
includer field become `T_NONE` with the following script:

```ruby
mod = Module.new do
  protected def foo = 1
end

klass = Class.new do
  include Module.new
  def run
    foo
  end
end

klass.include(mod)

GC.verify_compaction_references(double_heap: true, toward: :empty)

klass.new.run
```

I found a crash in a private application that this patch fixes, but
wasn't able to develop a small reproducer. Hence the above demo that
requires instrumentation.