Segfault when using `options.recover` and re-login fails or password was changed

[rhaas80]

imapfilter can fail with a segfault if one uses options.recover = true and the login fails or if one has changed the password in the lua account object in between initial login and the recover operation.

Pull request #253 has details.

A reproducers is something like this:

account = IMAP {
   server = 'outlook.office365.com',
   username = '[email protected]',
   password = "secretsecret"
}

print("re-login")
account:logout()
account:login()

print("logout and login")
account:logout()
account._account.password = "foobar"
account:login()