[jdk17u] RFR: 8275049: [ZGC] missing null check in ZNMethod::log_register

[Martin Doerr]

Clean backport of JDK-8275049.

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Commit messages:
 - Backport cf828673a9b76fd3f26db9a3f714166861a65c9e

Changes: https://git.openjdk.java.net/jdk17u/pull/195/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=195&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/jdk17u/pull/195.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/195/head:pull/195

PR: https://git.openjdk.java.net/jdk17u/pull/195