[N04] Replaces copying inherited code of transfer and transferFrom in LPToken

contracts/LPToken.sol

@@ -38,16 +38,12 @@ contract LPToken is ERC20Burnable, Ownable {
         _mint(recipient, amount);
     }
 
-    function transfer(address recipient, uint256 amount) public override returns (bool) {
-        swap.updateUserWithdrawFee(recipient, amount);
-        _transfer(_msgSender(), recipient, amount);
-        return true;
-    }
-
-    function transferFrom(address sender, address recipient, uint256 amount) public override returns (bool) {
-        swap.updateUserWithdrawFee(recipient, amount);
-        _transfer(sender, recipient, amount);
-        _approve(sender, _msgSender(), allowance(sender, _msgSender()).sub(amount, "ERC20: transfer amount exceeds allowance"));
-        return true;
+    /**
+     * @dev Overrides ERC20._beforeTokenTransfer() which get called on every transfers including
+     * minting and burning. This ensures that swap.updateUserWithdrawFees are called everytime.
+     */
+    function _beforeTokenTransfer(address from, address to, uint256 amount) internal override(ERC20) {
+        super._beforeTokenTransfer(from, to, amount);
+        swap.updateUserWithdrawFee(to, amount);
     }
 }

contracts/SwapUtils.sol

@@ -691,9 +691,6 @@ library SwapUtils {
 
         require(toMint >= minToMint, "Couldn't mint min requested LP tokens");
 
-        // Update msg.sender's withdraw fee
-        _updateUserWithdrawFee(self, msg.sender, toMint);
-
         // mint the user's LP tokens
         self.lpToken.mint(msg.sender, toMint);
 
@@ -723,6 +720,12 @@ library SwapUtils {
     }
 
     function _updateUserWithdrawFee(Swap storage self, address user, uint256 toMint) internal {
+
+        // If token is transferred to address 0 (or burned), don't update the fee.
+        if (user == address(0)) {
+            return;
+        }
+
         if (self.defaultWithdrawFee == 0) {
             // If current fee is set to 0%, set multiplier to FEE_DENOMINATOR
             self.withdrawFeeMultiplier[user] = FEE_DENOMINATOR;