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.h

@@ -21,7 +21,7 @@ struct channel_t {
     int           loop_count;
     int           record_pos;
     int           playback_pos;
-    int           active;
+    atomic_int    active;
     jack_port_t  *audio_in;
     jack_port_t  *audio_out;
 };