RFR: 8348420: Shenandoah: Check is_reserved before using ReservedSpace instances

[Y. Srinivas Ramakrishna]

On Fri, 24 Jan 2025 21:55:42 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Follow up on [JDK-8346572](https://bugs.openjdk.org/browse/JDK-8346572) for Shenandoah.

LGTM! 🚢

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Marked as reviewed by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23308#pullrequestreview-2573714199