Initialize variable to placate compiler.

Initialize variable to placate compiler.

I don't think there's a real problem here, because if we reach
the loop over 'tles' then we will either find at least one
TimeLineHistoryEntry such that oldest_segno != 0, in which case
unsummarized_lsn will be initialized, or else unsummarized_tli
will remain 0 and an error will occur before unsummarized_lsn
is used for anything. But some compilers are complainining, as
reported on list by Nathan Bossart and off-list by Andrew Dunstan.

Discussion: http://postgr.es/m/20231223215147.GA69623@nathanxps13