staging: dgrp: fix potential call to strncpy with a negative number

In dgrp_receive() there is:

   desclen = ((plen - 12) > MAX_DESC_LEN) ? MAX_DESC_LEN :
   	     	      	    plen - 12;
   strncpy(nd->nd_ps_desc, b + 12, desclen);

However, it's possible for plen to be <= 12 here so we'd be passing a
negative number into the strncpy().  Fix this to not make the strncpy
call and report an error if desclen is <= 0

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>