[jdk11u-dev] RFR: 8295872: [PPC64] JfrGetCallTrace: Need pc == nullptr check before frame constructor

[Martin Doerr]

Backport of JDK-[8295872](https://github.com/openjdk/jdk17u-dev/commit/3486bb2f8f89c6266e67b6efbc68dac14b2e6e74). `nullptr` needs to get replaced by `NULL` because 11 uses an older C++ standard.

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Commit messages:
 - Backport 3486bb2f8f89c6266e67b6efbc68dac14b2e6e74

Changes: https://git.openjdk.org/jdk11u-dev/pull/1519/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1519&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295872
  Stats: 18 lines in 2 files changed: 10 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1519.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1519/head:pull/1519

PR: https://git.openjdk.org/jdk11u-dev/pull/1519