fix: panic on connect to input

Upon inspection, it appears the received message will continuously send a single byte `11110000` when no keys are pressed. The length of this value is 1 byte. Not sure what this means or how MIDI works, but checking that
the input length is equal to 3 fixes this problem.

src/input.rs

@@ -241,6 +241,9 @@ impl Future for MidiInputTask {
                         &port,
                         self.settings.port_name,
                         move |stamp, message, _| {
+                            if message.len() != 3 {
+                                return;
+                            }
                             let _ = s.send(Reply::Midi(MidiData {
                                 stamp,
                                 message: [message[0], message[1], message[2]].into(),