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

Author: robertmhaas

src/backend/postmaster/walsummarizer.c

@@ -438,7 +438,7 @@ GetOldestUnsummarizedLSN(TimeLineID *tli, bool *lsn_is_exact,
 	LWLockMode	mode = reset_pending_lsn ? LW_EXCLUSIVE : LW_SHARED;
 	int			n;
 	List	   *tles;
-	XLogRecPtr	unsummarized_lsn;
+	XLogRecPtr	unsummarized_lsn = InvalidXLogRecPtr;
 	TimeLineID	unsummarized_tli = 0;
 	bool		should_make_exact = false;
 	List	   *existing_summaries;