CSMShadowNode: Fix error when disposed multiple times

[shotamatsuda]

This PR fixes CSMShadowNode throwing errors when disposed multiple times.

Disposing an instance multiple times may be invalid in itself, but it's better to be robust.

This happens when a CSMShadowNode is attached to a light's shadow, and the light and the CSM node are disposed independently.

const light = new DirectionalLight()
const csm = new CSMShadowNode()
light.shadow.shadowNode = csm

renderer.render() // Render a frame so that the CSM node is setup

light.dispose()
csm.dispose() // Error