mgw: do not fail MGCP on codec mismatch

Before this patch, when an CRCX+MDCX wants to set a codec list that has
no match with the codecs for the other conn of that same endpoint,
osmo-mgw returns an MGCP "FAIL" response.

When a client wants to change the codec, it has to do that one RTP port
at a time. So osmo-mgw *must* allow to configure an MGCP conn with a
codec choice that mismatches the other conn.

This is crucial to allow codec negotiation in osmo-msc: if MO has
already assigned a specific codec, and later wants to re-assign to the
codec that MT has chosen, the codec needs to be changed at osmo-mgw.

This patch is the minimal fix required to get re-assignment to a
different codec to work (via osmo-msc). There is more work to be done
about this bit of code in osmo-mgw, but keep that to a separate patch.

In detail, before this patch, we fail both
- when a side has no codecs,
- or when there is no single match between codecs of the two sides of
  the endpoint.
Remove only the second condition; after this patch, still fail when a
side has no codecs -- this allows mgcp_test.c to still pass.

Related: OS#6293
Related: osmo-msc I8760feaa8598047369ef8c3ab2673013bac8ac8a
Change-Id: I3d1163fe622bdd7dc42a485f796072524ab39db9

src/libosmo-mgcp/mgcp_codec.c

@@ -455,7 +455,17 @@ int mgcp_codec_decide(struct mgcp_conn_rtp *conn_src, struct mgcp_conn_rtp *conn
 		}
 	}
 
-	return -EINVAL;
+	if (conn_dst->end.codecs_assigned)
+		conn_dst->end.codec = &conn_dst->end.codecs[0];
+	else
+		return -EINVAL;
+
+	if (conn_src->end.codecs_assigned)
+		conn_src->end.codec = &conn_src->end.codecs[0];
+	else
+		return -EINVAL;
+
+	return 0;
 }
 
 /* Check if the codec has a specific AMR mode (octet-aligned or bandwith-efficient) set. */