[NETFILTER]: H.323 helper: fix endless loop caused by invalid TPKT len

When the TPKT len included in the packet is below the lowest valid value
of 4 an underflow occurs which results in an endless loop.

Found by testcase 0000058 from the PROTOS c07-h2250v4 testsuite.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_helper_h323.c

@@ -162,6 +162,8 @@ static int get_tpkt_data(struct sk_buff **pskb, struct ip_conntrack *ct,
 
 	/* Validate TPKT length */
 	tpktlen = tpkt[2] * 256 + tpkt[3];
+	if (tpktlen < 4)
+		goto clear_out;
 	if (tpktlen > tcpdatalen) {
 		if (tcpdatalen == 4) {	/* Separate TPKT header */
 			/* Netmeeting sends TPKT header and data separately */