RFR: JDK-8303047: avoid NULL after 8301661

[Matthias Baesken]

JDK-8301661 introduced a few usages of NULL in os_bsd.cpp and os_windows.cpp. This should be avoided and replaced by nullptr .

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Commit messages:
 - JDK-8303047

Changes: https://git.openjdk.org/jdk/pull/12709/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303047
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12709.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12709/head:pull/12709

PR: https://git.openjdk.org/jdk/pull/12709