Setting `DefaultFactory` to nil will cause a segv

Repro script:

```
$ ruby -Ilib -rmsgpack -e'MessagePack::DefaultFactory = nil; GC.start; "".to_msgpack'
```

This will cause a SEGV because [this global](https://github.com/msgpack/msgpack-ruby/blob/master/ext/msgpack/factory_class.c#L218) will get GC'd and the pointer will become bad.