RFR: 8335592: Fix -Wzero-as-null-pointer-constant warnings in RootNode ctor

[Tobias Hartmann]

On Wed, 3 Jul 2024 05:04:03 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change to the RootNode constructor. Both arguments
> to the LoopNode base class constructor are changed from 0 to nullptr. This
> removes some -Wzero-as-null-pointer-constant warnings when building with that
> enabled.
> 
> Testing: mach5 tier1

Looks good and trivial.

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19997#pullrequestreview-2155488626