Select random song for next round

Make sure, that a random song will be picked after a song was sung while a all songs challenge is running.

PartyModes/PartyModeChallenge/CPartyModeChallenge.cs

@@ -427,6 +427,7 @@ namespace VocaluxeLib.PartyModes.Challenge
                     _ScreenSongOptions.Sorting.Tabs = CBase.Config.GetTabs();
                     _ScreenSongOptions.Sorting.FilterPlaylistID = -1;
 
+                    _ScreenSongOptions.Selection.SongIndex = -1;
                     _ScreenSongOptions.Selection.CategoryIndex = -1;
                     _ScreenSongOptions.Selection.CategoryChangeAllowed = true;
                     break;