iptables: fix serializing multiple weekdays

Ref: https://bugs.openwrt.org/index.php?getfile=1053
Fixes: FS#1053
Fixes: 427bc12 ("iptables: fix serializing multiple weekdays")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

iptables.c

@@ -1208,7 +1208,7 @@ fw3_ipt_rule_time(struct fw3_ipt_rule *r, struct fw3_time *time)
 					rem--;
 				}
 
-				p += snprintf(p, rem, "%u", i);
+				len = snprintf(p, rem, "%u", i);
 
 				if (len < 0 || len >= rem)
 					break;