[geolocator_apple] Crash if listen/cancel is executed in quick succession

[mkurz-it]

The onDidAuthorizationChanged is executed by iOS somewhen afer CLLocationManager is created in onListenWithArguments.

If the the subscription is canceled in the meantime _eventSink (onCancelWithArguments) is set to nil leading to accessing _eventSink when already nil (BAD_ACCESS crash).

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