nat: Add size check for the payload

The msgb will always have these bytes but it is better practice
to verify that the message really has space for the two bytes.

openbsc/src/osmo-bsc_nat/bsc_nat.c

@@ -1185,7 +1185,7 @@ exit:
 		send_reset_ack(bsc);
 	} else if (parsed->ipa_proto == IPAC_PROTO_IPACCESS) {
 		/* do we know who is handling this? */
-		if (msg->l2h[0] == IPAC_MSGT_ID_RESP) {
+		if (msg->l2h[0] == IPAC_MSGT_ID_RESP && msgb_l2len(msg) > 2) {
 			struct tlv_parsed tvp;
 			int ret;
 			ret = ipa_ccm_idtag_parse(&tvp,