WT_ITEM wasn't being cleared, so if the key was not instantiated we
dropped core.  Format reads every key it inserts, and that action was
instantiating the keys, now we're not instantiating the keys, we hit
the bug.
Use the local variable, don't repeat the retrieval (the compiler should
figure it all out, but it still looks wrong).
Another bug caused by an uninitialized WT_ITEM structure combined with an
uninitialized key, found because all keys are no longer instantiated during
a test/format run.
Restructure the code to avoid instantiating the key until it's necessary.