TSL: Fixed persistent reference of the first stack

[sunag]

Fixes #31644

Description

It's a simple fix, but it should fix the memory leak issue regarding the first stack.

Before	Now

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.1	338.91 79.1	+0 B +0 B
WebGPU	577.1 159.18	577.1 159.18	-2 B +4 B
WebGPU Nodes	575.71 158.93	575.71 158.94	-2 B +4 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	470.7 113.91	470.7 113.91	+0 B +0 B
WebGPU	647.66 175.14	647.65 175.14	-2 B +2 B
WebGPU Nodes	601.76 164.31	601.76 164.31	-2 B +2 B