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

[Johan Sjölen]

On Wed, 25 Jan 2023 11:45:47 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

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

Passes tier1.

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