fix disappearing backpack during trade

[Zbizu]

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

prevent a trade that would result in one of trade containers disappearing in situation when both players try to trade equipped backpack

expected result: the player who accepts the trade should see "there is not enough room" error

Issues addressed: #3742

Notes

needs testing

[ramon-bernardo]

That solves it, but shouldn't we find bug in internalMoveItem?

[Zbizu]

That solves it, but shouldn't we find bug in internalMoveItem?

Adding item to container normally terminates the trade (or at least it should). In this situation trade was already completing so it couldn't terminate. The item moves by cloning and deleting itself, but the other item was added before the container deleted itself so the item was lost. I believe there won't be any more problems after very specific situation which is trade got resolved.