mgcp_send_dummy: Check RTP destination is available before attempt tx

Several log messages showing "Failed to send dummy RTP packet." were
seen in a osmo-mgw on the field. Let's re-use the function to check and
provide more information on what's wrong to ease debugging.

Related: SYS#5498
Change-Id: Iee6ac1f4d24c131e3bf40c37e6fdc252e5208ec8
diff --git a/tests/mgcp/mgcp_test.c b/tests/mgcp/mgcp_test.c
index e6862e5..5bb7b27 100644
--- a/tests/mgcp/mgcp_test.c
+++ b/tests/mgcp/mgcp_test.c
@@ -634,6 +634,10 @@
 		dummy_packets += 1;
 	}
 
+	/* Make sure address+port are valid */
+	OSMO_ASSERT(dest_host);
+	OSMO_ASSERT(dest_port);
+
 	return len;
 }