idl: add to_null attribute to the spoolss formname array

idl: add to_null attribute to the spoolss formname array

OpenPrinterEx requests have been observed in the wild carrying a device
mode formname "A4" followed by non-utf16 garbage after the null
terminator. Such requests currently fail during unmarshalling in the
ndr_pull_charset() codepath, causing intermittent print job failures.

This change ensures that garbage after the device mode formname null
terminator is not processed in unmarshalling.

https://bugzilla.samba.org/show_bug.cgi?id=8606

Signed-off-by: Jeremy Allison <jra@samba.org>