RFR: 8337523: Fix -Wzero-as-null-pointer-constant warnings in jvmci code

[Kim Barrett]

On Wed, 31 Jul 2024 08:27:56 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Please review this trivial change that removes some uses of literal 0 as a
>> null pointer constant in jvmci code.
>> 
>> Testing: mach5 tier1
>
> Looks good and trivial.

Thanks for reviews @chhagedorn and @shipilev

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PR Comment: https://git.openjdk.org/jdk/pull/20399#issuecomment-2260499797