Fix play queue index handling in SyncPlay

Three index bugs in PlayQueueManager, two of which leave
PlayingItemIndex out of bounds, making every subsequent Buffering/Ready
request throw and leaving the group unusable until it empties:

- RemoveFromPlaylist did not compensate for removed items preceding the
  playing item: removing the playing item together with earlier items
  could select the wrong item or crash with an out-of-bounds index.
- Next/Previous on an empty playlist with RepeatOne/RepeatAll reported
  success or set PlayingItemIndex to 0 on an empty list, crashing
  downstream in Group and corrupting the index.
- SetPlayingItemByIndex accepted an index equal to the playlist count
  (latent off-by-one, callers currently pre-validate).

MediaBrowser.Controller/SyncPlay/Queue/PlayQueueManager.cs

@@ -272,7 +272,7 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
         public void SetPlayingItemByIndex(int playlistIndex)
         {
             var playlist = GetPlaylistInternal();
-            if (playlistIndex < 0 || playlistIndex > playlist.Count)
+            if (playlistIndex < 0 || playlistIndex >= playlist.Count)
             {
                 PlayingItemIndex = NoPlayingItemIndex;
             }
@@ -293,6 +293,20 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
         {
             var playingItem = GetPlayingItem();
 
+            // Removed items that precede the playing item shift its index as well.
+            var removedBeforePlayingItem = 0;
+            if (playingItem is not null)
+            {
+                var playlist = GetPlaylistInternal();
+                for (var index = 0; index < PlayingItemIndex; index++)
+                {
+                    if (playlistItemIds.Contains(playlist[index].PlaylistItemId))
+                    {
+                        removedBeforePlayingItem++;
+                    }
+                }
+            }
+
             _sortedPlaylist.RemoveAll(item => playlistItemIds.Contains(item.PlaylistItemId));
             _shuffledPlaylist.RemoveAll(item => playlistItemIds.Contains(item.PlaylistItemId));
 
@@ -303,12 +317,12 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
                 if (playlistItemIds.Contains(playingItem.PlaylistItemId))
                 {
                     // Playing item has been removed, picking previous item.
-                    PlayingItemIndex--;
+                    PlayingItemIndex -= removedBeforePlayingItem + 1;
                     if (PlayingItemIndex < 0)
                     {
                         // Was first element, picking next if available.
                         // Default to no playing item otherwise.
-                        PlayingItemIndex = _sortedPlaylist.Count > 0 ? 0 : NoPlayingItemIndex;
+                        PlayingItemIndex = GetPlaylistInternal().Count > 0 ? 0 : NoPlayingItemIndex;
                     }
 
                     return true;
@@ -444,6 +458,11 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
         /// <returns><c>true</c> if the playing item changed; <c>false</c> otherwise.</returns>
         public bool Next()
         {
+            if (GetPlaylistInternal().Count == 0)
+            {
+                return false;
+            }
+
             if (RepeatMode.Equals(GroupRepeatMode.RepeatOne))
             {
                 LastChange = DateTime.UtcNow;
@@ -474,6 +493,11 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
         /// <returns><c>true</c> if the playing item changed; <c>false</c> otherwise.</returns>
         public bool Previous()
         {
+            if (GetPlaylistInternal().Count == 0)
+            {
+                return false;
+            }
+
             if (RepeatMode.Equals(GroupRepeatMode.RepeatOne))
             {
                 LastChange = DateTime.UtcNow;