0.12.0 regression: Selector::wait_timeout on an empty Selector hangs forever

I have some code that conditionally spawns channels, and needs to block waiting for any of them to have a message ready, or timeout. Every time a channel is spawned, it's added to the selector via `Selector::recv`. But because the channels are conditionally added, the selector may be empty. My expectation is that trying to wait on an empty Selector returns some sort of error immediately. `Timeout` would be fine here, or a new `SelectError::Empty` variant to indicate that there's nothing to select upon. This test passes on flume 0.11.1, but hangs forever on 0.12.0. ``` #[test] fn wait_timeout_empty_selector_times_out() { // Nothing is attached to this Selector. // The only reasonable thing here is to timeout either immediately or let result = flume::Selector::new() .wait_timeout(Duration::from_millis(10)); assert!(result.is_err()); } ```