Repair still another bug in the btree page split WAL reduction patch:

Repair still another bug in the btree page split WAL reduction patch:
it failed for splits of non-leaf pages because in such pages the first
data key on a page is suppressed, and so we can't just copy the first
key from the right page to reconstitute the left page's high key.
Problem found by Koichi Suzuki, patch by Heikki.