Update getAccountBalances method in Kraken to safely unrwarp arrayOfCryptoBalances

[maakle]

Hey,

you should update the getAccountBalances method to this once here and safely unwrap arrayOfCryptoBalances since it will lead to a fatal crash in case a user has once set up the API correctly but then deletes the keys on the website and comes back to the App. This here at least won't lead to the App crashing:

public func getAccountBalances(completion: @escaping (ResponseType) -> Void) {
            let apiType = Kraken.API.getAccountBalance
            if apiType.checkInterval(response: store.balanceResponse) {
                completion(.cached)
            } else {
                krakenDataTaskFor(api: apiType) { (response) in
                    
                    guard let json = response.json as? Dictionary<String, Any> else {
                        print("Error: Cast Failed in \(#function)")
                        return
                    }
                    if let arrayOfCryptoBalances = json["result"] as? Dictionary<String,String> {
                        var balances: [Balance] = []
                        
                        for cryptoBalance in arrayOfCryptoBalances {
                            let newBalance = ["type": cryptoBalance.key,
                                              "amount": cryptoBalance.value,
                                              "available": cryptoBalance.value]
                            
                            balances.append(Balance(json: newBalance, currencyStore: self))
                        }
                        
                        self.store.balances = balances
                        self.store.balanceResponse = response.httpResponse
                        completion(.fetched)
                    }
                }.resume()
            }
        }

[trsathya]

This is fixed in #19