Fix compiler warning about return -1 in a void function.

src/control/audio.c

@@ -191,7 +191,7 @@ void libvlc_audio_set_track( libvlc_input_t *p_input, int i_track,
     int i;
 
     if( !p_input_thread )
-        return -1;
+        return;
 
     var_Change( p_input_thread, "audio-es", VLC_VAR_GETCHOICES, &val_list, NULL );
     for( i = 0; i < val_list.p_list->i_count; i++ )