Initialize wal_level in the initial checkpoint record.

As per Coverity and Tom Lane, commit 402b586 (back-patched to v17
as 2b5819e) forgot to initialize this new structure member in this
code path.

Author: robertmhaas

src/backend/access/transam/xlog.c

@@ -5045,6 +5045,7 @@ BootStrapXLOG(void)
 	checkPoint.ThisTimeLineID = BootstrapTimeLineID;
 	checkPoint.PrevTimeLineID = BootstrapTimeLineID;
 	checkPoint.fullPageWrites = fullPageWrites;
+	checkPoint.wal_level = wal_level;
 	checkPoint.nextXid =
 		FullTransactionIdFromEpochAndXid(0, FirstNormalTransactionId);
 	checkPoint.nextOid = FirstGenbkiObjectId;