RFR: 8337784: Fix simple -Wzero-as-null-pointer-constant warnings in linux/posix code

[Kim Barrett]

Please review this trivial change that replaces some uses of literal 0 as a
null pointer constant in linux/posix code to instead use nullptr.

Testing: mach5 tier1

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Commit messages:
 - fix simple linux/posix

Changes: https://git.openjdk.org/jdk/pull/20452/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20452&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337784
  Stats: 6 lines in 3 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20452.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20452/head:pull/20452

PR: https://git.openjdk.org/jdk/pull/20452