audio: add NULL check for SR recorder

If the SR recorder cannot be created for some reason, we currently just
print "Ready!" on the display. Improve UX by informing the user about
the problem.

main/audio.c

@@ -851,6 +851,13 @@ static esp_err_t start_rec(void)
     }
     free(audio_codec);
     free(wake_word);
+
+    if (cfg_ar.sr_handle == NULL) {
+        ESP_LOGE(TAG, "failed to init SR recorder");
+        ui_pr_err("Recorder init failed", "Check logs");
+        return ESP_FAIL;
+    }
+
     hdl_ar = audio_recorder_create(&cfg_ar);
 
     return ret;