RFR: 8373411: Crash when PrintSharedArchiveAndExit is enabled but shared heap is disabled

[Stefan Karlsson]

Found this while poking around at Valhalla that turned off heap sharing. The fix is simple, there's a missing HeapShared::is_loading() check that we missed when refactoring the object streaming code.

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Commit messages:
 - 8373411: Crash when PrintSharedArchiveAndExit is enabled but shared heap is disabled

Changes: https://git.openjdk.org/jdk/pull/28741/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373411
  Stats: 46 lines in 2 files changed: 45 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28741.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28741/head:pull/28741

PR: https://git.openjdk.org/jdk/pull/28741