RFR: 8335294: Fix simple -Wzero-as-null-pointer-constant warnings in gc code

[Thomas Schatzl]

On Fri, 28 Jun 2024 03:44:53 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change that replaces some uses of literal 0 as a null
> pointer constant in gc code to instead use nullptr.
> 
> There is also one place where the use was eliminated entirely, because it was
> dead code following a [[noreturn]] call.
> 
> Testing: mach5 tier1.

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/19934#pullrequestreview-2147806879