library: f_diameter_xceive(): fix no matching alt

Currently an unexpected PDU will trigger a DTE (no matching alt).
Let's add an additional wildcard alt and terminate grecefully.

Change-Id: I6dbc646d5f036f454f197137373796f40c4c9e74
Related: SYS#5602, SYS#6333
diff --git a/library/DIAMETER_Emulation.ttcn b/library/DIAMETER_Emulation.ttcn
index 09f92be..d9d9f7a 100644
--- a/library/DIAMETER_Emulation.ttcn
+++ b/library/DIAMETER_Emulation.ttcn
@@ -260,6 +260,10 @@
 	DIAMETER.send(t_DIAMETER_Send(g_diameter_conn_id, tx));
 	alt {
 	[] DIAMETER.receive(tr_DIAMETER_RecvFrom_R(rx_t)) -> value mrf { }
+	[] DIAMETER.receive(tr_DIAMETER_RecvFrom_R(?)) -> value mrf {
+		setverdict(fail, "Rx unexpected DIAMETER PDU: ", mrf);
+		mtc.stop;
+		}
 	[] DIAMETER.receive(tr_SctpAssocChange) { repeat; }
 	[] DIAMETER.receive(tr_SctpPeerAddrChange)  { repeat; }
 	[] T.timeout {