usbaudio: triage panic from splash()

Do the check in gui_wps_show() to avoid calling splash()
from anything other than the main thread

Change-Id: Icc1c6f287d82dc6e73b7cae9c75c18be73b82d48

apps/gui/wps.c

···
 #include "skin_engine/wps_internals.h"
 #include "open_plugin.h"
 
+#ifdef USB_ENABLE_AUDIO
+#include "usbstack/usb_audio.h"
+#endif
+
 #define FF_REWIND_MAX_PERCENT 3 /* cap ff/rewind step size at max % of file */
                                 /* 3% of 30min file == 54s step size */
 #define MIN_FF_REWIND_STEP 500
···
  */
 long gui_wps_show(void)
 {
+/* NOTE: if USBAudio ever gets its own DSP channel, this block can go away! */
+#ifdef USB_ENABLE_AUDIO
+    if (usb_audio_get_active())
+    {
+        splash(HZ*2, ID2P(LANG_USB_DAC_ACTIVE));
+    }
+#endif
     long button = 0;
     bool restore = true;
     bool exit = false;

apps/playback.c

···
 static void audio_start_playback(const struct audio_resume_info *resume_info,
                                  unsigned int flags)
 {
-/* NOTE: if USBAudio ever gets its own DSP channel, this block can go away! */
+/* 
+ * Refuse to start playback if usb audio is active. See gui_wps_show() for
+ * a splash message to the user.
+ * NOTE: if USBAudio ever gets its own DSP channel, this block can go away!
+ */
 #ifdef USB_ENABLE_AUDIO
     if (usb_audio_get_active())
     {
-        splash(HZ*2, ID2P(LANG_USB_DAC_ACTIVE));
         queue_reply(&audio_queue, 0);
         return;
     }