RFR: 8275049: [ZGC] missing null check in ZNMethod::log_register

[Martin Doerr]

The VM crashes while trying to read (*p)->klass() in "ZNMethod::log_register" on PPC64. We need a null check. See JBS for details.

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Commit messages:
 - 8275049: [ZGC] missing null check in ZNMethod::log_register

Changes: https://git.openjdk.java.net/jdk/pull/5892/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5892&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275049
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5892.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5892/head:pull/5892

PR: https://git.openjdk.java.net/jdk/pull/5892