Function safe_close 

pub fn safe_close(
    account: &AccountView,
    destination: &AccountView,
) -> ProgramResult

Close account, sending all its lamports to destination.

Steps:

1. Overflow-checked addition of lamports into destination.
2. Zero account lamports.
3. close_unchecked(): zeros data and clears the owner field.

Both accounts must be writable - caller is responsible for that check.

Safety

Caller must guarantee no active borrows exist on account at call time.