RFR: 8256370: Add asserts to Reference.getInactive()

[Roman Kennke]

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.

Testing:
 - [x] tier1
 - [x] tier2

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Commit messages:
 - 8256370: Add asserts to Reference.getInactive()

Changes: https://git.openjdk.java.net/jdk/pull/1231/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1231&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256370
  Stats: 4 lines in 2 files changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1231.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1231/head:pull/1231

PR: https://git.openjdk.java.net/jdk/pull/1231