FV-ANC-10-CL2 Operations on accounts marked as closed

Bad

// No checks for closed accounts in other program functions.
let data = ctx.accounts.account.try_borrow_data()?;

Good

let data = ctx.accounts.account.try_borrow_data()?;
let discriminator: [u8; 8] = data[..8].try_into()?;
if discriminator == *b"CLOSEDAC" {
    return Err(ProgramError::InvalidAccountData); // Prevent usage of closed account.
}