sink-input, source-output: set sink/source to NULL before the "unlink post" hook

sink-input, source-output: set sink/source to NULL before the "unlink post" hook

At the time the "unlink post" hook is fired, the stream is not any more
connected to its old device, so it makes sense to reset the sink/source
pointer to NULL before firing the hook. If this is not done, the pointer
may become stale during the "unlink post" hook, because
module-bluetooth-policy does a card profile change in its "unlink post"
callback, so even if the pointer is valid when module-bluetooth-policy's
callback is called, it will be invalid in subsequent callbacks.