Exception when using MemoryUtil in JDK 9

```
Exception in thread "main" java.lang.NullPointerException
	at com.github.pyknic.immutablearray.internal.util.MemoryUtil.clear(MemoryUtil.java:31)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at com.github.pyknic.immutablearray.internal.ints.IntImmutableArrayBuilder.build(IntImmutableArrayBuilder.java:90)
	at com.snabbdb.MainApp.main(MainApp.java:91)
```