RFR: 8344320: GenShen: Possible null pointer usage in shGenerationalHeapFix plab nullptr

[Kelvin Nilsen]

Certain threads, including the vm thread and Shenandoah's control thread, do not have PLABs.  There are scenarios where each may need to allocate old-gen memory on the slow-path of the load reference barrier.  This PR fixes an assumption in the allocator that assumed all threads have plabs.

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Commit messages:
 - Only dereference plab if plab != nullptr
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 17 more: https://git.openjdk.org/shenandoah/compare/8bc2dff6...b21c47e3

Changes: https://git.openjdk.org/shenandoah/pull/543/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=543&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344320
  Stats: 17 lines in 1 file changed: 1 ins; 1 del; 15 mod
  Patch: https://git.openjdk.org/shenandoah/pull/543.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/543/head:pull/543

PR: https://git.openjdk.org/shenandoah/pull/543