Repair boundary-case bug introduced by patch of two months ago that

fixed incorrect initial setting of StartUpID.  The logic in XLogWrite()
expects that Write->curridx is advanced to the next page as soon as
LogwrtResult points to the end of the current page, but StartupXLOG()
failed to make that happen when the old WAL ended exactly on a page
boundary.  Per trouble report from Hannu Krosing.