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).

0 files changed, 0 insertions, 0 deletions