RFR: Check that pointer is in heap before checking the affiliation of owning region

[William Kemper]

This is a small bug fix that was meant to go in with the concurrent remembered set scanning changes

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Commit messages:
 - Check that pointer is in heap before checking the affiliation of owning region

Changes: https://git.openjdk.java.net/shenandoah/pull/52/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=52&range=00
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/52.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/52/head:pull/52

PR: https://git.openjdk.java.net/shenandoah/pull/52