VLCMediaListPlayer: Set the media player's media

Set the media player's media when the playItemAtNumber method is
called.

Closes #669

Sources/VLCMediaListPlayer.m

@@ -255,7 +255,11 @@ static void HandleMediaListPlayerStopped(const libvlc_event_t * event, void * se
 
 - (void)playItemAtNumber:(NSNumber *)index
 {
-    libvlc_media_list_player_play_item_at_index(instance, [index intValue]);
+    dispatch_async(_libVLCBackgroundQueue, ^{
+        VLCMedia *media = [_mediaList mediaAtIndex:[index intValue]];
+        _mediaPlayer.media = media;
+        libvlc_media_list_player_play_item_at_index(instance, [index intValue]);
+    });
 }
 
 - (void)setRepeatMode:(VLCRepeatMode)repeatMode