Fix out-of-bounds read past template fields terminator

BUG: KASAN: slab-out-of-bounds in netflow_scan_and_export+0x719/0x990 [ipt_NETFLOW]
Read of size 2 at addr ffff88802e84d53d by task kworker/0:1/34

Note that this is not critical bug, since it reads slack data and it's not
used.

Author: aabc

ipt_NETFLOW.c

@@ -4166,11 +4166,12 @@ static void export_dev(struct net_device *dev)
 	/* encode all fields */
 	for (i = 0; ; ) {
 		int type = tpl->fields[i++];
-		int size = tpl->fields[i++];
+		int size;
 		int n;
 
 		if (!type)
 			break;
+		size = tpl->fields[i++];
 		switch (type) {
 		case observationDomainId:
 			put_unaligned_be32(engine_id, ptr);