Fix incorrect variable datatype

Both datatypes map to the same underlying one which is why it still
worked, but we should use the correct type.

Author: Kyotaro HORIGUCHI

src/backend/access/transam/xlog.c

@@ -9263,7 +9263,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 	/* then check whether slots limit removal further */
 	if (max_replication_slots > 0 && keep != InvalidXLogRecPtr)
 	{
-		XLogRecPtr	slotSegNo;
+		XLogSegNo	slotSegNo;
 
 		XLByteToSeg(keep, slotSegNo);