Check account exists in vault.state before accessing it.

[0xhipo]

Fixed #303

Description:
This pull request addresses the issue where the locked_balance function aborts when called on a fresh balance_manager that has not interacted with the pool. The solution involves adding a check for locked_balance before placing an order.

Changes Made:

• Updated pool::locked_balance, check account exists before accessing.
• Updated deepbook::master_tests::test_locked_balance to include check_locked_balance<SUI, USDC>(ALICE, pool1_id, alice_balance_manager_id, &alice_locked_balance, &mut test); before placing an order.

Testing:

• The updated test case now passes, confirming the fix.
• Verified that the computational load increase is minimal and acceptable.

[0xaslan]

Can we put this if statement in the beginning of the function, and keep the remaining code the same?

/// Returns the locked balance for the balance_manager in the pool.
/// Locked balance = open orders + settled balances.
/// Returns (base_quantity, quote_quantity, deep_quantity).
public fun locked_balance<BaseAsset, QuoteAsset>(
    self: &Pool<BaseAsset, QuoteAsset>,
    balance_manager: &BalanceManager,
): (u64, u64, u64) {
    let account_orders = self.get_account_order_details(balance_manager);
    let self = self.load_inner();
    if (!self.state.account_exists(balance_manager.id())) {
        return (0, 0, 0)
    };

    let mut base_quantity = 0;
    let mut quote_quantity = 0;
    let mut deep_quantity = 0;

    account_orders.do_ref!(|order| {
        let maker_fee = self.state.history().historic_maker_fee(order.epoch());
        let (base, quote, deep) = order.locked_balance(maker_fee);
        base_quantity = base_quantity + base;
        quote_quantity = quote_quantity + quote;
        deep_quantity = deep_quantity + deep;
    });

    let settled_balances = self
        .state
        .account(balance_manager.id())
        .settled_balances();
    base_quantity = base_quantity + settled_balances.base();
    quote_quantity = quote_quantity + settled_balances.quote();
    deep_quantity = deep_quantity + settled_balances.deep();

    (base_quantity, quote_quantity, deep_quantity)
}