Add asserts to Reference.getInactive()

A follow-up to JDK-8256106, this is adding two asserts to check that the API is used as it should be, i.e. only on inactive FinalReferences. Also, in Finalizer, where getInactive() is used, there is a null-check. The GC must never clean the referent, and Java code doesn't clean it either, it would be a bug if we ever see null there. I think it's better to fail there (with assert or NPE) when that happens instead of silently accepting it.