netfilter: conntrack: fix bug in for_each_sctp_chunk

skb_header_pointer() will return NULL if offset + sizeof(_sch) exceeds
skb->len, so this offset < skb->len test is redundant.

if sch->length == 0, this will end up in an infinite loop, add a check
for sch->length > 0

Fixes: 9fb9cbb1082d ("[NETFILTER]: Add nf_conntrack subsystem.")
Suggested-by: Florian Westphal <[e-mail]>
Signed-off-by: Sriram Yagnaraman <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_proto_sctp.c

···
 
 #define for_each_sctp_chunk(skb, sch, _sch, offset, dataoff, count)	\
 for ((offset) = (dataoff) + sizeof(struct sctphdr), (count) = 0;	\
-	(offset) < (skb)->len &&					\
-	((sch) = skb_header_pointer((skb), (offset), sizeof(_sch), &(_sch)));	\
+	((sch) = skb_header_pointer((skb), (offset), sizeof(_sch), &(_sch))) &&	\
+	(sch)->length;	\
 	(offset) += (ntohs((sch)->length) + 3) & ~3, (count)++)
 
 /* Some validity checks to make sure the chunks are fine */