[NETFILTER]: xt_sctp: fix endless loop caused by 0 chunk length

Fix endless loop in the SCTP match similar to those already fixed in
the SCTP conntrack helper (was CVE-2006-1527).
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/xt_sctp.c

@@ -62,7 +62,7 @@ match_packet(const struct sk_buff *skb,
 
 	do {
 		sch = skb_header_pointer(skb, offset, sizeof(_sch), &_sch);
-		if (sch == NULL) {
+		if (sch == NULL || sch->length == 0) {
 			duprintf("Dropping invalid SCTP packet.\n");
 			*hotdrop = 1;
 			return 0;