RFR: 8293998: [PPC64] JfrGetCallTrace: assert(_pc != nullptr) failed: must have PC

[Martin Doerr]

JfrGetCallTrace can still observe last_Java_pc == 0 (see JBS for more details). We can just skip the sample in this very rare case.

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Commit messages:
 - 8293998: [PPC64] JfrGetCallTrace: assert(_pc != nullptr) failed: must have PC

Changes: https://git.openjdk.org/jdk/pull/10339/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10339&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293998
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/10339.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10339/head:pull/10339

PR: https://git.openjdk.org/jdk/pull/10339