Failed select should unselect

[horejsek]

From RFC https://tools.ietf.org/html/rfc1730#section-6.3.1:

 Only one mailbox may be selected at a time in a session;
 simultaneous access to multiple mailboxes requires multiple
 sessions.  The SELECT command automatically deselects any
 currently selected mailbox before attempting the new selection.
 Consequently, if a mailbox is selected and a SELECT command that
 fails is attempted, no mailbox is selected.