look-up by (subscriber, protocol, transaction_id) tuple

... rather than the previous incomplete (lchan, transaction_id) tuple
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index a16b0de..a5c562f 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -3544,7 +3544,7 @@
 	}
 	
 	/* Find transaction */
-	trans = trans_find_by_id(lchan, transaction_id);
+	trans = trans_find_by_id(lchan->subscr, GSM48_PDISC_CC, transaction_id);
 
 	DEBUGP(DCC, "(bts %d trx %d ts %d ti %x sub %s) "
 		"Received '%s' from MS in state %d (%s)\n",