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>

src/channel.c

@@ -23,7 +23,7 @@ void channel_add(jack_client_t *client, int idx)
         return;
     }
 
-    channels[idx].active = 1;
+    atomic_store(&channels[idx].active, 1);
     atomic_store(&channels[idx].state, STATE_IDLE);
     channels[idx].prev_state = -1;
     channels[idx].loop_count = 0;
@@ -38,6 +38,6 @@ void channel_remove(jack_client_t *client, int idx)
 {
     jack_port_unregister(client, channels[idx].audio_in);
     jack_port_unregister(client, channels[idx].audio_out);
-    channels[idx].active = 0;
+    atomic_store(&channels[idx].active, 0);
     channel_count--;
 }