Core/Cleanup: remove C-style cast to (Bag*):

* add ToBag() methods for Item class;
* add GetBagByPos() method for Player class.

src/server/game/Server/Protocol/Handlers/AuctionHouseHandler.cpp

@@ -188,7 +188,7 @@ void WorldSession::HandleAuctionSellItem(WorldPacket & recv_data)
         return;
     }
 
-    if (it->IsBag() && !((Bag*)it)->IsEmpty())
+    if (!it->IsNotEmptyBag())
     {
         SendAuctionCommandResult(0, AUCTION_SELL_ITEM, AUCTION_INTERNAL_ERROR);
         return;

src/server/game/Server/Protocol/Handlers/ItemHandler.cpp

@@ -516,7 +516,7 @@ void WorldSession::HandleSellItemOpcode(WorldPacket & recv_data)
         }
 
         // prevent sell non empty bag by drag-and-drop at vendor's item list
-        if (pItem->IsBag() && !((Bag*)pItem)->IsEmpty())
+        if (pItem->IsNotEmptyBag())
         {
             _player->SendSellError(SELL_ERR_CANT_SELL_ITEM, pCreature, itemguid, 0);
             return;
@@ -668,7 +668,7 @@ void WorldSession::HandleBuyItemInSlotOpcode(WorldPacket & recv_data)
     {
         for (int i = INVENTORY_SLOT_BAG_START; i < INVENTORY_SLOT_BAG_END; ++i)
         {
-            if (Bag *pBag = (Bag*)_player->GetItemByPos(INVENTORY_SLOT_BAG_0,i))
+            if (Bag* pBag = _player->GetBagByPos(i))
             {
                 if (bagguid == pBag->GetGUID())
                 {

src/server/game/Server/Protocol/Handlers/MailHandler.cpp

@@ -219,7 +219,7 @@ void WorldSession::HandleSendMail(WorldPacket & recv_data)
             return;
         }
 
-        if (item->IsBag() && !((Bag*)item)->IsEmpty())
+        if (item->IsNotEmptyBag())
         {
             pl->SendMailResult(0, MAIL_SEND, MAIL_ERR_EQUIP_ERROR, EQUIP_ERR_CAN_ONLY_DO_WITH_EMPTY_BAGS);
             return;