fix: guard against NULL audio ports and defer channel activation until port registration succeeds

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

src/looper.c

@@ -35,6 +35,12 @@ int process_callback(jack_nframes_t nframes, void *arg)
     for (int c = 0; c < MAX_CHANNELS; c++) {
         if (!channels[c].active) continue;
 
+        /* Guard against NULL ports (e.g. if port registration failed) */
+        if (!channels[c].audio_in || !channels[c].audio_out) {
+            fprintf(stderr, "WARN: channel %d has NULL audio port(s), skipping\n", c);
+            continue;
+        }
+
         jack_default_audio_sample_t *in  = (jack_default_audio_sample_t *)
             jack_port_get_buffer(channels[c].audio_in, nframes);
         jack_default_audio_sample_t *out = (jack_default_audio_sample_t *)