RFR: 8315890: Attempts to load from nullptr in instanceKlass.cpp and unsafe.cpp

[Matias Saavedra Silva]

Calls in instanceKlass.cpp and unsafe.cpp try to call an atomic load on method calls that could return nullptr. This patch ensures that nullptr is not passed into the load. Verified with tier1-5 tests.

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Commit messages:
 - 8315890: Attempts to load from nullptr in instanceKlass.cpp and unsafe.cpp

Changes: https://git.openjdk.org/jdk/pull/16405/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16405&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315890
  Stats: 4 lines in 2 files changed: 2 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16405.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16405/head:pull/16405

PR: https://git.openjdk.org/jdk/pull/16405