RFR: 8295872: [PPC64] JfrGetCallTrace: Need pc == nullptr check before frame constructor

[Martin Doerr]

The check pc == nullptr is needed before the frame constructor.

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Commit messages:
 - 8295872: [PPC64] JfrGetCallTrace: Need pc == nullptr check before frame constructor

Changes: https://git.openjdk.org/jdk/pull/10846/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295872
  Stats: 12 lines in 2 files changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10846.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10846/head:pull/10846

PR: https://git.openjdk.org/jdk/pull/10846