Change imap_conn_find() to always return an authenticated conn.

Change imap_conn_find() to always return an authenticated conn.

With the flag MUTT_IMAP_CONN_NONEW, it was already ensured the
connection would be authenticated.  However, without that flag, an
error in opening the connection or authentication would still return
an idata.

The callers that didn't bother to check the state were still assuming
authenticated, because they were all subseqeuently issuing an
"authenticated state" command to the server.

Rather than add state checks to every caller, just change the function
to return NULL if the idata did not end up in an authenticated state.

Remove the now redundant state checks in imap_open_mailbox() and
imap_get_mailbox().