SIP re-login fetches correct auth session

After a login session expires and the SIP server logs in again to
acquire a new authtoken, be sure to use the new authtoken when fetching
the new session object.  Otherwise, the login_session is left undef and
you may see errors like this when checking in items:

Can't call method "ws_ou" on an undefined value at
/usr/local/share/perl/5.10.1/OpenILS/SIP/Transaction/Checkin.pm line 77.

Incidentally, the checkin code is the only code that references
$sip_handler->{login_session}.  And the problem listed above only occurs
when the login session expires just prior to a checkin call.  In any
other scenario, the login_session would ultimately get repaired.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>