fix: add null-checks for MIDI ports and use atomic access for channel active flag

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

tests/integration.c

@@ -34,6 +34,7 @@ static unsigned char midi_inject_velocity = 0;
 
 static int midi_inject_process(jack_nframes_t nframes, void *arg) {
     (void)arg;
+    if (!midi_inject_port) return 0;
     void *port_buf = jack_port_get_buffer(midi_inject_port, nframes);
     if (!port_buf) return 0;
     jack_midi_clear_buffer(port_buf);