RFR: JDK-8301069: Replace NULL with nullptr in share/libadt/

[Johan Sjölen]

Do the conversion in the share/libadt/ sub-directory and all of its files.

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Commit messages:
 - Fix comment alignment
 -  Replace NULL with nullptr in share/libadt/

Changes: https://git.openjdk.org/jdk/pull/12184/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12184&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301069
  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12184.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12184/head:pull/12184

PR: https://git.openjdk.org/jdk/pull/12184