RFR: 8337418: Fix -Wzero-as-null-pointer-constant warnings in prims code

[Aleksey Shipilev]

On Tue, 30 Jul 2024 04:12:33 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

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

All right, this looks fine. (I am somewhat allergic to `{}` syntax, but it is what it is.)

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20385#pullrequestreview-2207285678