orb: fix when orb_subscribe_multi is called before orb_advertise_multi

This fixes the previously introduced unit test. It fixes the case where
orb_subscribe_multi is called multiple times with different instances,
and no publisher advertised the topic yet. In this case all subscribers
got the same instance 0.

src/modules/uORB/uORBManager.cpp

@@ -273,6 +273,9 @@ int uORB::Manager::node_open
 
 		/* open the path as either the advertiser or the subscriber */
 		fd = px4_open(path, advertiser ? PX4_F_WRONLY : PX4_F_RDONLY);
+
+	} else {
+		*instance = 0;
 	}
 
 	/* we may need to advertise the node... */