BTree_deleteNextBucket(): This failed to decref the temp result from
Tim Peters authored
BTree_lastBucket().  In unusual cases, this could cause a chain of buckets
to leak (the DegenerateBTree tests appeared to be the only ones that
provoked this, and there it leaked 285 IISet buckets).  Other uses of
BTree_lastBucket() appear to be refcount-correct.
ded90255