s3-printing: fix crash in printer_list_set_printer()

s3-printing: fix crash in printer_list_set_printer()

The printer list database format was recently changed to accommodate for
the printcap location field.
One of the tdb_pack calls is not provided with a location string
argument, this causes a crash on some platforms.

https://bugzilla.samba.org/show_bug.cgi?id=8762

Signed-off-by: Günther Deschner <gd@samba.org>
Signed-off-by: Jim McDonough <jmcd@samba.org>
Signed-off-by: Lars Müller <lars@samba.org>
Autobuild-User: David Disseldorp <ddiss@samba.org>
Autobuild-Date: Wed Feb 15 19:34:38 CET 2012 on sn-devel-104