Instantiate a full key for the parent ref of a page during in-memory splits

The original page's key may have been truncated and may not be valid after the split.

Also, take care to free the original key if it was instantiated, fixing a leak found by address sanitizer.

refs #1419, #1421
@michaelcahill, I'm missing this one -- a suffix compressed key must be shorter than the key on the split page we're creating, can you give me an example of when it wouldn't be valid?