services/pipewire: avoid use-after-free in PwDefaultTracker::reset()

[bbedward]

relevant to #534

onFatalError() is queued, so by the time reset() runs the PwNode objects may already be destroyed. The setter
functions go through setSimpleObjectHandle → QObject::disconnect() on dangling pointers → SIGSEGV.

This changes to directly null the pointers and emit signals in reset(), skipping the disconnect-from-dead-objects path.

[outfoxxed]

Shouldn't this already be handled by the QObject::destroyed connection in SimpleObjectHandle? (I am not particularly proud of that interface, which should probably be replaced by bindables, but it should theoretically work for this)

[bbedward]

You're right but somehow something causes mDefaultSink to point to freed memory in these crashes, but I dont see a case off hand why the destroyed signal wouldnt work - I'll look more into it.

[bbedward]

Alternative fix pushed with per-handle connections, should still fix the crash - which I believe is caused by the nodes sharing the same destroyed slot.