RFR: 8275049: [ZGC] missing null check in ZNMethod::log_register [v2]

[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.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Use Atomic::load to read the oop and change klass name to N/A in case of null oop.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5892/files
  - new: https://git.openjdk.java.net/jdk/pull/5892/files/bf1a79e1..5f48b723

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5892&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5892&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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