RFR: 8320886: Unsafe_SetMemory0 is not guarded

[Jorn Vernee]

See JBS issue.

Guard the memory access done in Unsafe_SetMemory0 to prevent a SIGBUS error from crashing the VM when a truncated memory mapped file is accessed.

Testing: local `InternalErrorTest`, Tier 1-5 (ongoing)

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Commit messages:
 - fix
 - add test

Changes: https://git.openjdk.org/jdk/pull/16848/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16848&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320886
  Stats: 18 lines in 2 files changed: 11 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/16848.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16848/head:pull/16848

PR: https://git.openjdk.org/jdk/pull/16848