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

@@ -76,12 +76,13 @@ class GroupStage(TournamentSubModel):
         else:
             previous = self.tournament.get_previous_live_group_stages(self.step - 1)
             for gs in previous:
-                team_registration = gs.teams[self.index].team_registration
-                if team_registration in gs_teams:
-                    team = gs_teams[team_registration]
-                else:
-                    team = GroupStageTeam(team_registration)
-                    gs_teams[team_registration] = team
+                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]
+                    else:
+                        team = GroupStageTeam(team_registration)
+                        gs_teams[team_registration] = team
 
         # compute matches
         for match in self.matches.all():