Lack of zero address check throughout the codebase could lead to unwanted redeployments, address(0) ownership and `onTokenTransfer` unsuccessful.  
Audit: 2023-04-frankencoin-findings
Severity: MEDIUM

# Lines of code

https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/MintingHub.sol#L88-L91
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/MintingHub.sol#L54-L57
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/MintingHub.sol#L124
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/Position.sol#L50-L68
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/Position.sol#L78
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/StablecoinBridge.sol#L26-L31
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/Frankencoin.sol#L125-L128
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/Frankencoin.sol#L83-L89
https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/Equity.sol#L93-L95


# Vulnerability details

## Impact
User defined address should always have zero address check. This checks SHOULD NOT BE MISSED IN CASE OF A FACTORY CONTRACT. 
This will lead to redeployments of contract and blockage of certain functionality as described below. 
It is also worth to note that is Data Validation bug was acknowledged by Uniswap v3 and modified. Check here - \
https://github.com/Uniswap/v3-core/blob/main/audits/tob/audit.pdf 

## Proof of Concept
In `constructor()` there is no check for zero address. Once it's set to any address, there's no configuration functions to change it.
```
    constructor(address _owner, address _hub, address _zchf, address _collateral, 
        uint256 _minCollateral, uint256 _initialLimit, uint256 initPeriod, uint256 _duration,
        uint256 _challengePeriod, uint32 _mintingFeePPM, uint256 _liqPrice, uint32 _reservePPM) {
        require(initPeriod >= 3 days); // must be at least three days, recommended to use higher values
        setOwner(_owner);
        original = address(this);
        hub = _hub;
        price = _liqPrice;
        zchf = IFrankencoin(_zchf);
        collateral = IERC20(_collateral);
        mintingFeePPM = _mintingFeePPM;
        reserveContribution = _reservePPM;
        minimumCollateral = _minCollateral;
        challengePeriod = _challengePeriod;
        start = block.timestamp + initPeriod; // one week time to deny the position
        cooldown = start;
        expiration = start + _duration;
        limit = _initialLimit;
``` 

Similarly, [`onTokenTransfer()` in StableCoinBridge](https://github.com/code-423n4/2023-04-frankencoin/blob/1022cb106919fba963a89205d3b90bf62543f68f/contracts/StablecoinBridge.sol#L75) if constructor address are not set properly and set to address(0), then this function will fail, indefinitely.
```