Add more sanity-checking to PageAddItem and PageIndexTupleDelete,

Add more sanity-checking to PageAddItem and PageIndexTupleDelete,
to prevent spreading of corruption when page header pointers are bad.
Merge PageZero into PageInit, since it was never used separately, and
remove separate memset calls used at most other PageInit call points.
Remove IndexPageCleanup, which wasn't used at all.