incorrect WAL segno calculation

[gsmolk]

archive show in json format:
https://gist.github.com/gsmol/cbad57b4d2ef97ebbe93e75137779f5a

Actual archive state:
https://gist.github.com/gsmol/4d6ad74000204befa7dd727ee5b8d8d8

Fixed:
https://gist.github.com/gsmol/b49b1bce7147a935cce25b671f75a8c7

Reported by Alexander NIkitin

[gsmolk]

Fixed.
TODO: test with lots of WAL segments