fix sms layer timer leak/corruiption using trans_free()

we only stopped the cp1* timer on reception of cp-ack, but not
in error cases like rx of cp-error.
diff --git a/openbsc/src/transaction.c b/openbsc/src/transaction.c
index 3825de1..5e55665 100644
--- a/openbsc/src/transaction.c
+++ b/openbsc/src/transaction.c
@@ -92,6 +92,9 @@
 	case GSM48_PDISC_CC:
 		_gsm48_cc_trans_free(trans);
 		break;
+	case GSM48_PDISC_SMS:
+		_gsm411_sms_trans_free(trans);
+		break;
 	}
 
 	if (trans->lchan)