RFR: 8295872: [PPC64] JfrGetCallTrace: Need pc == nullptr check before frame constructor [v2]

[Martin Doerr]

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

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

  Remove acquire barrier again. The null check is the clean fix.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10846/files
  - new: https://git.openjdk.org/jdk/pull/10846/files/fe0bfd32..53f5fc06

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10846&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10846&range=00-01

  Stats: 10 lines in 2 files changed: 4 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