Reset MIDI-control when a given ctrl is not available

This fixes an issue with "/route/eq/freq/0 S1":
When a newly select strip that does not have a EQ
(e.g. mixbus or master), the control from the previously
selected strip is used.  -- Reported by tavasti on IRC.

libs/surfaces/generic_midi/midicontrollable.cc

@@ -270,6 +270,7 @@ MIDIControllable::lookup_controllable()
 	boost::shared_ptr<Controllable> c = _surface->lookup_controllable (_current_uri);
 
 	if (!c) {
+		set_controllable (boost::shared_ptr<PBD::Controllable>());
 		return -1;
 	}