Quick fix of issue #106.

Sorry for introducing the problem of issue #106! I've fixed it in this pull request and the project-template tests now pass. The problem was that `required` could in pathological cases be given input where the first byte not satisfying `isContinuation` actually isn't a valid UTF-8 character start byte.

I still don't fully understand why the test cases in conduit did not catch this problem but I will try to construct a new test case and submit in a following pull request. For now, however this patch is safe in the sense that any input which isn't explicitly correct will be passed to `splitSlowly`.