sndio: fix crash when volume is adjusted while stopped

Set volume/mute function pointers when handle is freed, to ensure
sio_setvol() is not called on a freed handle. Fixes crashes when volume
is adjusted while playback is stopped.

Fixes #25796

modules/audio_output/sndio.c

@@ -208,6 +208,8 @@ static void Stop (audio_output_t *aout)
 {
     aout_sys_t *sys = aout->sys;
 
+    aout->volume_set = NULL;
+    aout->mute_set = NULL;
     sio_close (sys->hdl);
 }