Core/Battlegrounds: Free instance id in the proper place.

fixes #966

src/server/game/Battlegrounds/BattlegroundMgr.cpp

@@ -111,9 +111,6 @@ void BattlegroundMgr::Update(uint32 diff)
                 if (!m_ClientBattlegroundIds[i][bg->GetBracketId()].empty())
                     m_ClientBattlegroundIds[i][bg->GetBracketId()].erase(bg->GetClientInstanceID());
 
-                // Free up the instance id and allow it to be reused
-                sMapMgr->FreeInstanceId(bg->GetInstanceID());
-
                 delete bg;
             }
         }

src/server/game/Maps/MapInstanced.cpp

@@ -245,9 +245,15 @@ bool MapInstanced::DestroyInstance(InstancedMaps::iterator &itr)
         // so in the next map creation, (EnsureGridCreated actually) VMaps will be reloaded
         Map::UnloadAll();
     }
+
+    // Free up the instance id and allow it to be reused for bgs and arenas (other instances are handled in the InstanceSaveMgr)
+    if (itr->second->IsBattlegroundOrArena())
+        sMapMgr->FreeInstanceId(itr->second->GetInstanceId());
+
     // erase map
     delete itr->second;
     m_InstancedMaps.erase(itr++);
+
     return true;
 }