RFR: 8337785: Fix simple -Wzero-as-null-pointer-constant warnings in x86 code

[Albert Mingkun Yang]

On Sat, 3 Aug 2024 23:26:25 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change that replaces some uses of literal 0 as a
> null pointer constant in x86 code to instead use nullptr.
> 
> Testing: mach5 tier1

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/20453#pullrequestreview-2218234346