RFR: Only verify last object start for marked objects [v2]

[Kelvin Nilsen]

On Fri, 3 Dec 2021 22:39:10 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> In some cases, verification could invoke `size` on an unmarked object resulting in a crash.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Explicitly track last marked object when verifying last object start

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.java.net/shenandoah/pull/103