RFR: JDK-8301070: Replace NULL with nullptr in share/memory/

[Johan Sjölen]

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

-------------

Commit messages:
 - Fix manual issue
 -  Replace NULL with nullptr in share/memory/

Changes: https://git.openjdk.org/jdk/pull/12185/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301070
  Stats: 678 lines in 65 files changed: 0 ins; 0 del; 678 mod
  Patch: https://git.openjdk.org/jdk/pull/12185.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12185/head:pull/12185

PR: https://git.openjdk.org/jdk/pull/12185