VestingContract doesn't make funds above vestingTotalAmount immediately available.

[tomkha]

New issue checklist

• I have read all of the README
• I have searched existing issues and this is not a duplicate.

General information

• Library version(s): 1.5.4
• Browser version(s):
• Devices/Simulators/Machine affected:
• Reproducible in the testnet? (Yes/No): Yes
• Related issues:

Bug report

VestingContract.getMinCap must not return a value greater than vestingTotalAmount

Expected behavior

Funds above vestingTotalAmount must be immediately available after creation.

Actual behavior

Extra funds are unlocked in steps preceding vestingStart block.

Steps to reproduce

Create a vesting contract with the vesting start block somewhere in the future, define vesting step block, vesting step amount, total vesting amount. The transaction must have a value greater than total vesting amount. If you try to withdraw those extra funds before the vesting start block you'll get "balance error".

[tomkha]

Here is a simplified demo:

const Nimiq = require('@nimiq/core');

const balance = Nimiq.Policy.coinsToLunas(1500);
const start = 1000;
const stepBlocks = 100;
const stepAmount = Nimiq.Policy.coinsToLunas(100);
const totalAmount = Nimiq.Policy.coinsToLunas(1000);

const contract = new Nimiq.VestingContract(balance, Nimiq.Address.NULL, start, stepBlocks, stepAmount, totalAmount);


for (let block = 0; block <= 2000; block += 100) {
    const minCap = contract.getMinCap(block);
    const available = Math.max(0, balance - minCap);
    console.log(`#${block}: min cap: ${Nimiq.Policy.lunasToCoins(minCap)}, available: ${Nimiq.Policy.lunasToCoins(available)}`);
}

and the result:

#0: min cap: 2000, available: 0
#100: min cap: 1900, available: 0
#200: min cap: 1800, available: 0
#300: min cap: 1700, available: 0
#400: min cap: 1600, available: 0
#500: min cap: 1500, available: 0
#600: min cap: 1400, available: 100
#700: min cap: 1300, available: 200
#800: min cap: 1200, available: 300
#900: min cap: 1100, available: 400
#1000: min cap: 1000, available: 500
#1100: min cap: 900, available: 600
#1200: min cap: 800, available: 700
#1300: min cap: 700, available: 800
#1400: min cap: 600, available: 900
#1500: min cap: 500, available: 1000
#1600: min cap: 400, available: 1100
#1700: min cap: 300, available: 1200
#1800: min cap: 200, available: 1300
#1900: min cap: 100, available: 1400
#2000: min cap: 0, available: 1500

in other words, getMinCap counts negative steps too.

Here is the contract from the testnet:
https://test.nimiq.watch/#NQ50+NE4C+8X7C+HFA5+7F58+DVPC+VC6C+UVEH+GE47