netfilter: nf_conntrack_h323: fix off-by-one in DecodeQ931

This patch corrects an off-by-one error in the DecodeQ931 function in
the nf_conntrack_h323 module. This error could result in reading off
the end of a Q.931 frame.
Signed-off-by: Toby DiPasquale <toby@cbcg.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_h323_asn1.c

@@ -846,9 +846,10 @@ int DecodeQ931(unsigned char *buf, size_t sz, Q931 *q931)
 	sz -= len;
 
 	/* Message Type */
-	if (sz < 1)
+	if (sz < 2)
 		return H323_ERROR_BOUND;
 	q931->MessageType = *p++;
+	sz--;
 	PRINT("MessageType = %02X\n", q931->MessageType);
 	if (*p & 0x80) {
 		p++;