Silence uninitialized-variable warning.

Quite a few buildfarm animals are warning about this, and lapwing
is actually failing (because -Werror).  It's a false positive AFAICS,
so no need to do more than zero the variable to start with.

Discussion: https://postgr.es/m/YYXJnUxgw9dZKxlX@paquier.xyz

src/backend/access/transam/xlog.c

@@ -12109,6 +12109,9 @@ read_backup_label(XLogRecPtr *checkPointLoc, TimeLineID *backupLabelTLI,
 	uint32		hi,
 				lo;
 
+	/* suppress possible uninitialized-variable warnings */
+	*checkPointLoc = InvalidXLogRecPtr;
+	*backupLabelTLI = 0;
 	*backupEndRequired = false;
 	*backupFromStandby = false;