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

[William Kemper]

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

-------------

Commit messages:
 - Exit during initialization if required memory cannot be reserved

Changes: https://git.openjdk.org/jdk/pull/23308/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23308&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348420
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23308.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23308/head:pull/23308

PR: https://git.openjdk.org/jdk/pull/23308