staging: dgrp: check for a valid proc dir entry pointer

while proc_create fails, the register_proc_table can do a derefernce of the
null pointer causing to oops the system, instead check for a valid pointer
at register and unregister

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>