accounts-db: throw error explicitly for next_account_offset calculation when overflows

[HaoranYi]

Problem

1. When we scanning append_vec, next_account_offset calculation is not safe, and it assumes that calculation never overflow. If it overflows, the app will crash. Fortunately, this is only an issue for tests and debug tools. For production validator code, we always sanitize the storage when we load. Therefore, we should (hopefully)never hit overflow for new_account_offset calculation. However, it would still be nice to throw the error explicitly when we overflow.
2. We don't have unit test to cover scan_index. And the current test coverage doesn't cover corner cases.
   

Summary of Changes

1. throw overflow error explicitly when calculating next_account_offset.
2. add test to cover scan_index with the corner cases, such incomplete account's data and garbage data that cause overflow. (now split into Accounts-db: Add scan tests to cover bad accounts storage files #2123)
3. improve test_scan_pubkey to cover mroe corner cases (now split into Accounts-db: Add scan tests to cover bad accounts storage files #2123).

Fixes #

[brooksprumo]