Fixed bug which would occur when deleting account with pending changes

Fixed bug which would occur when deleting account with pending changes

- When the selected account was NULL,
  accounts_dialog_has_pending_change() would not check its state
  correctly. That function is working with a pointer on a pointer to the
  account and was not dereferencing properly.
- The pointer to the account we pass to
  accounts_dialog_has_pending_change() is now properly initialized to
  NULL.
- When deleting an account, the COL_ACCOUNT_POINTER column is now
  properly set to NULL.