[NETFILTER]: SCTP conntrack: fix crash triggered by packet without chunks

When a packet without any chunks is received, the newconntrack variable
in sctp_packet contains an out of bounds value that is used to look up an
pointer from the array of timeouts, which is then dereferenced, resulting
in a crash. Make sure at least a single chunk is present.

Problem noticed by George A. Theall <theall@tenablesecurity.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_proto_sctp.c

@@ -254,7 +254,7 @@ static int do_basic_checks(struct ip_conntrack *conntrack,
 	}
 
 	DEBUGP("Basic checks passed\n");
-	return 0;
+	return count == 0;
 }
 
 static int new_state(enum ip_conntrack_dir dir,

net/netfilter/nf_conntrack_proto_sctp.c

@@ -261,7 +261,7 @@ static int do_basic_checks(struct nf_conn *conntrack,
 	}
 
 	DEBUGP("Basic checks passed\n");
-	return 0;
+	return count == 0;
 }
 
 static int new_state(enum ip_conntrack_dir dir,