Issue Description:

Hello! When I have an object create a Key-Value Observing observer for changes to that object's property's property (i.e. using a compound key path), no matter how I try to stop observing it, the deallocated observer ends up being notified of subsequent changes. In the best case this causes a crash. For example, running the following (from a file via swift bug.swift or in a Playground):

import Foundation

class Message: NSObject {
    @objc dynamic var delivered: Bool = false
}

class Emissary: NSObject {
    @objc dynamic var message: Message
    
    var observation: NSKeyValueObservation?
    
    init(message: Message) {
        self.message = message
        super.init()
        
        observation = observe(\.message.delivered, changeHandler: {
            object, change in
            print("object", object, "changed:", change)
        })
    }
   
    deinit {
        observation?.invalidate()
        observation = nil
        print("goodbye from", self)
    }
}

let message = Message()
var emissary: Emissary? = Emissary(message: message)

emissary = nil

message.delivered = true // crash here

results in a crash at the indicated line. I see the "goodbye from" get printed, and I see the expected lack of printing from the change handler, but the program does not exit cleanly. The exact crash and stack trace can differ depending on what the address of the deallocated observer points to, but it includes _NSSetCharValueAndNotify and some private NSKeyValueObserving methods.

I first noticed this issue when using Combine's NSObject.KeyValueObservingPublisher, but narrowed it down to Swift's KVO observer. Using the old-style observeValue(forKeyPath:of:change:context:) works in the scenario I show above. Is this meant to be a supported configuration for Swift's KVO observer?