Fix oops in cifs_strfromUCS_le mounting to servers which do not specify their OS

Fix oops in cifs_strfromUCS_le mounting to servers which do not specify their OS

commit 69765529d701c838df19ea1f5ad2f33a528261ae upstream.

Fixes kernel bug #10451 http://bugzilla.kernel.org/show_bug.cgi?id=10451

Certain NAS appliances do not set the operating system or network operating system
fields in the session setup response on the wire.  cifs was oopsing on the unexpected
zero length response fields (when trying to null terminate a zero length field).

This fixes the oops.

Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>