The function burn is used by users to burn an option position by minting the required liquidity and unlocking the collateral.
As how the function is designed right now in order to do that, the user needs to send his shares to the contract balance.
This is simply too risky, as anyone can call the function and basically burn the shares deposited by the users, before they even get the chance to call the function first.
Instead of the need to send the shares to the contract balance, the function can be refactored to check the balance of shares the user posses and to burn them in the moment of execution or on top of that to input a uint value of how many shares the user wants to burn. 
