fix: upgrade panel not properly comparing item for cancel

The Upgrade Panel was just exiting any time GC objects got
updated. We can instead use the ItemDefIndex because
upgrades persist across items of the same item def.

This fixes an issue where party invites could be abused
to prevent players from upgrading items in MvM, and it
also fixes a bug where unrelated inventory updates would
always close you out of the menu, as well as lobby updates
(players disconnecting/reconnecting)

ref: ValveSoftware/Source-1-Games#4288

Author: mastercoms

src/game/client/tf/player_vs_environment/c_tf_upgrades.cpp

@@ -716,7 +716,7 @@ void CHudUpgradePanel::PlayerInventoryChanged( C_TFPlayer *pPlayer )
 				continue;
 
 			// If the item has changed since the panel was created, cancel out of the upgrade screen
-			if ( pItemSlotBuyPanel->GetItemID() != pCurrentItem->GetItemID() )
+			if ( pItemSlotBuyPanel->GetItemID() != pCurrentItem->GetItemDefIndex() )
 			{
 				OnCommand( "cancel" );
 				return;
@@ -1149,7 +1149,7 @@ void CHudUpgradePanel::UpdateUpgradeButtons( void )
 				CEconItemView *pCurItemData = CTFPlayerSharedUtils::GetEconItemViewByLoadoutSlot( m_hPlayer, pItemSlotBuyPanel->nSlot );
 				if ( pCurItemData )
 				{
-					pItemSlotBuyPanel->SetItemID( pCurItemData->GetItemID() );
+					pItemSlotBuyPanel->SetItemID( pCurItemData->GetItemDefIndex() );
 				}
 
 				if ( !m_bInspectMode )