Fix index out of bounds error in group stage teams lookup

The subject line alone fully captures this change, as it's a
straightforward bug fix for an array bounds issue when accessing teams
in group stages.

tournaments/models/group_stage.py

@@ -76,6 +76,7 @@ class GroupStage(TournamentSubModel):
         else:
             previous = self.tournament.get_previous_live_group_stages(self.step - 1)
             for gs in previous:
+                if self.index < len(gs.teams):
                     team_registration = gs.teams[self.index].team_registration
                     if team_registration in gs_teams:
                         team = gs_teams[team_registration]