*fix arena join if start rating is set to 0

note: rating == 0 is used by BattlegroundQueue for update stuff

--HG--
branch : trunk

src/game/BattleGroundHandler.cpp

@@ -701,6 +701,9 @@ void WorldSession::HandleBattlemasterJoinArena(WorldPacket & recv_data)
             arenaRating = avg_pers_rating;
     }
 
+    if (arenaRating <= 0)
+        arenaRating = 1;
+
     BattleGroundQueue &bgQueue = sBattleGroundMgr.m_BattleGroundQueues[bgQueueTypeId];
     if (asGroup)
     {