RFR: Filter out invalid pointers picked up by old gen SATB

[William Kemper]

In some cases, the SATB barrier used to support old generation marking may capture a reference to an object that is later evacuated. Old gen marking will crash when it tries to mark through the reference.

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Commit messages:
 - Filter out invalid pointers picked up by old gen SATB

Changes: https://git.openjdk.java.net/shenandoah/pull/140/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=140&range=00
  Stats: 16 lines in 1 file changed: 15 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/140.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/140/head:pull/140

PR: https://git.openjdk.java.net/shenandoah/pull/140