engine: stop jack processing on mutex poison

crates/engine/src/jack/jack_client.rs

@@ -85,7 +85,11 @@ impl Jack {
                 // implements audio and MIDI input and output on a realtime basis.
                 ClosureProcessHandler::new(Box::new({
                     let app = app.clone();
-                    move|c: &_, s: &_|app.write().unwrap().process(c, s)
+                    move|c: &_, s: &_|if let Ok(mut app) = app.write() {
+                        app.process(c, s)
+                    } else {
+                        Control::Quit
+                    }
                 }) as BoxedAudioHandler<'j>),
             )?;
             *self.state.write().unwrap() = Active(client);