Correct an old logic error in btree page splitting: when considering a split

Correct an old logic error in btree page splitting: when considering a split
exactly at the point where we need to insert a new item, the calculation used
the wrong size for the "high key" of the new left page.  This could lead to
choosing an unworkable split, resulting in "PANIC: failed to add item to the
left sibling" (or "right sibling") failure.  Although this bug has been there
a long time, it's very difficult to trigger a failure before 8.2, since there
was generally a lot of free space on both sides of a chosen split.  In 8.2,
where the user-selected fill factor determines how much free space the code
tries to leave, an unworkable split is much more likely.  Report by Joe
Conway, diagnosis and fix by Heikki Linnakangas.