RFR: 8335663: Fix simple -Wzero-as-null-pointer-constant warnings in C2 code

[Kim Barrett]

On Thu, 4 Jul 2024 07:17:36 GMT, Julian Waters <jwaters at openjdk.org> wrote:

>> Please review this trivial change that replaces some uses of literal 0 as a null
>> pointer constant in C2 code to instead use nullptr.
>> 
>> Testing: mach5 tier1
>
> Marked as reviewed by jwaters (Committer).

Thanks for reviews @TheShermanTanker and @chhagedorn

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PR Comment: https://git.openjdk.org/jdk/pull/20022#issuecomment-2208597506