GSCON: avoid sending connection oriented data when not connected

When no connection is present and had never existed, then
conn->sccp.msc is unpopulated. However, there may be situations where
osmo_bsc_sigtran_send() is executed while no connection is present.

At the moment we assert on conn->sccp.msc, which would cause osmo-bsc
to exit. In order to avoid this, better check conn->sccp.msc and drop
the sccp message when the check is negative.

- Remove assertion, add check.

Change-Id: I4eaa983702224e5995a388ea9890ee04212eb569
Related: OS#3446

src/osmo-bsc/osmo_bsc_sigtran.c

@@ -348,7 +348,12 @@ int osmo_bsc_sigtran_send(struct gsm_subscriber_connection *conn, struct msgb *m
 
 	OSMO_ASSERT(conn);
 	OSMO_ASSERT(msg);
-	OSMO_ASSERT(conn->sccp.msc);
+
+	if (!conn->sccp.msc) {
+		LOGP(DMSC, LOGL_ERROR, "MSC is not connected. Dropping.\n");
+		msgb_free(msg);
+		return -EINVAL;
+	}
 
 	msc = conn->sccp.msc;