Regression in sodium-0.11.0.1

In the commit preceding my [pull request](https://github.com/kentuckyfriedtakahe/sodium/pull/17), c870fefadfff95355d692c44298438dd13fc5d04, the following short program terminates immediately and successfully:

    import Control.Applicative
    import FRP.Sodium

    main :: IO ()
    main = sync $ do
        hold 0 (switchE (pure never))
        return ()

In sodium-0.11.0.1, the same program terminates with "undefined".