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

[Mandy Chung]

On Sun, 22 Nov 2020 22:15:20 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Thanks for adding this asserts.  If there is any reason to use `getInactive` by other references in the future, we could remove these asserts at that time.
>
> I didn't notice this before it was integrated.
> 
> The test for inactive isn't right; rather than `next == this` it
> should be `next != null`.  This becomes apparent once
> FinalizerHistogram is fixed to call getInactive() rather than get().
> 
> I noticed this while working on JDK-8256517, where I ran into some
> similar issues.  I will address these problems as part of that change.

@kimbarrett  thanks for correcting this inactive FinalReference assert.

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