spoolss: fix segfault when "default devmode" is disabled

spoolss: fix segfault when "default devmode" is disabled

Currently when "default devmode" is explicitly disabled, and a printer
is added with a null device mode, spoolssd crashes in copy_devicemode().

Both construct_printer_info2() and construct_printer_info8() code paths
currently unconditionally attempt to copy a printers device mode,
without checking whether one is present.

This change fixes this regression such that construct_printer_info*()
functions check for a null device mode before copying.

https://bugzilla.samba.org/show_bug.cgi?id=9433

Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Nov 29 13:03:05 CET 2012 on sn-devel-104
(cherry picked from commit 2e12deedcfdc5ce3637a125b083b0f00b208bf61)