Fix regression in 4.14 in spools server

https://bugzilla.samba.org/show_bug.cgi?id=14635

I tried to enable the existing tests in source4/torture/rpc/spoolss.c that currently rely on OS intalled driver files, but it seems the tests fail subtly in a myriad of ways when enabled, so I'm giving up after a day as it's probably going to take at least a week until all tests pass in master.