staging: dgrp: check for NULL pointer in (un)register_proc_table

register_proc_table and unregister_proc_table didn't deal with the
possibility that the *table pointer could be NULL.  Check for this and
return if table is NULL.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>