issue: nbtpage.c,_bt_pagedel may get wrong result

/*
* To avoid deadlocks, we'd better drop the leaf page lock
* before going further.
*/
_bt_unlockbuf(rel, leafbuf);

/*
* Check that the left sibling of leafbuf (if any) is not
* marked with INCOMPLETE_SPLIT flag before proceeding
*/
Assert(leafblkno == scanblkno);
if (_bt_leftsib_splitflag(rel, leftsib, leafblkno))
{
    ReleaseBuffer(leafbuf);
    return;
}