Clear class component refs on unmount in compat

[contactjawad]

An object ref on a class component isn't reset to null when the component unmounts (preact/compat):

const ref = createRef()
// render <Foo ref={ref} />, then unmount Foo
ref.current // still the old Foo instance, expected null

In unmount() the ref is only cleared when r.current is falsy or equals the vnode's DOM node. For a compat class component the ref points at the component instance (vnode._component), so neither case matches and the stale instance is left behind. DOM refs and function refs are already cleared — it's just the instance case slipping through.

I added r.current == vnode._component to that guard. Ran the refs suite too to check nothing regressed. Test added.

[github-actions]

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