omldummy: Suppress RSL transmission errors

In omldummy, we don't have a RSL link and hence any transmission
attempts to RSL should silently go to /dev/null rather than printing
error messages via libosmo-abis.  So check for the OMLDUMMY variant and
silently discard any RSL message that the code may want to transmit.

Change-Id: I04a9ec7e416822cc1e45c533e6a66628689615ad

src/common/abis.c

@@ -81,6 +81,13 @@ static void drain_oml_queue(struct gsm_bts *bts)
 
 int abis_bts_rsl_sendmsg(struct msgb *msg)
 {
+	OSMO_ASSERT(msg->trx);
+
+	if (msg->trx->bts->variant == BTS_OSMO_OMLDUMMY) {
+		msgb_free(msg);
+		return 0;
+	}
+
 	/* osmo-bts uses msg->trx internally, but libosmo-abis uses
 	 * the signalling link at msg->dst */
 	msg->dst = msg->trx->rsl_link;