Properly terminate Partials in sinkGet when the upstream terminates

`Get` parsers that rely on `Get.isolate` or `Applicative.many` to terminate fail to parse in `sinkGet` with this error message:

    Data/Conduit/Cereal.hs:48:36-56: Irrefutable pattern failed for pattern Data.Serialize.Get.Fail msg

The fix is rather simple.