Fixed users can't withdraw funds after calling claimFixedPremium upfront

Summary

Fixed depositors will have their claim tokens set to zero after claiming their premium, which will cause a revert on their withdrawals.

Root Cause

In vault.sol, the logic of setting fixedClaimToken[msg.sender] to zero during the claimFixedPremium function prevents fixed depositors from withdrawing their funds afterward.

Firstly in the deposit function, fixed depositor gets minted claim tokens. L359
```
// Mint claim tokens
  fixedClaimToken[msg.sender] += shares;
```
Secondly in the claimFixedPremium function, fixed depositor burns his claim tokens for upfront premium. User got minted bearer tokens and his claimToken balance is set to 0.
```
// Burn claim tokens
fixedClaimToken[msg.sender] = 0;
```

As third and last, fixed depositor tries to withdraw funds. However, a claimToken balance check in below withdraw function prevents him to withdraw his funds.

if (side == FIXED) {
    require(fixedToVaultNotStartedWithdrawalRequestIds[msg.sender].length == 0, "WAR");

    // need to have claim tokens
    uint256 claimBalance = fixedClaimToken[msg.sender];
    //uint256 claimBalance = fixedClaimToken.balanceOf(msg.sender);
    require(claimBalance > 0, "NCT"); //@audit claimBalance is already set to 0 when claiming the premium, this call will revert

Internal pre-conditions

A fixed depositor needs to have a non zero balance of fixedClaimToken to call claimFixedPremium. --> After depositing that check will pass
The fixed depositor then will call claimFixedPremium to have his premium upfront, sourced by variable users. When claimed, claimTokens will be burned and user balance will be set to 0.
The fixed depositor then will try to withdraw his funds. However, withdraw function checks if user claimToken balance is bigger than 0.
Since it has been set to 0 in claimFixedPremium, withdraw call will revert for fixed user.

External pre-conditions

none

Attack Path

There is no attack. I identified the flow above.

Impact

Fixed user funds are stuck, unable to withdraw.

PoC

https://github.com/sherlock-audit/2024-08-saffron-finance/blob/main/lido-fiv/contracts/LidoVault.sol#L358-L360 https://github.com/sherlock-audit/2024-08-saffron-finance/blob/main/lido-fiv/contracts/LidoVault.sol#L412-L414 https://github.com/sherlock-audit/2024-08-saffron-finance/blob/main/lido-fiv/contracts/LidoVault.sol#L428-L434

Mitigation

No response

sherlock-audit / 2024-08-saffron-finance-judging

Motomoto - Fixed users can't withdraw funds after calling claimFixedPremium upfront #164