Item headers are written from the front of the block to the tail.
Item values are written from the tail of the block towards the head.
The math to detect their overlapping in the center forgot to take the
length of the item header into account. We could have final item
headers and values overriding each other which causes file data to
appear as an item header.
Signed-off-by: Zach Brown <zab@versity.com>