torture: add zero length FSCTL_SRV_COPYCHUNK test

Windows Server 2012 returns NT_STATUS_INVALID_PARAMETER for
FSCTL_SRV_COPYCHUNK requests that include a server-side copy length of
zero, in line with MS-SMB2 3.3.5.15.6.

We should match this behaviour, so test for it.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>