Crash in timsieved's cmd_authenticate() on 2.4.6

[Florian Pflug]

Hi

I've just found a bug in timsieved's cmd_authenticate on cyrus 2.4.6.

If the authenticating user is an admin, we proceed even if the mailbox
lookup fails. In this case, the mboxlist_lookup seems to leave the
mboxlist_entry uninitialized, making the code believe that the mailbox
is remote if the bit MBTYPE_REMOTE happens to be set in mbentry.mvtype.
The crash then happens when xstrdup tried to copy mbentry.partition.

Initializing mbentry to zero in cmd_authenticate() fixes the bug and
allows admins without mailboxes (like root) to authenticate again
on my system.

best regards,
Florian Pflug