ccdpack: Fix some pointer comparisons

It seems that on my Mac (at least):

    if ( ++c >= retbuf + BUFLENG ) {

comes out true even when `c` and `retbuf` differ by 1 with `c`
less than `retbuf`. It looks like this is some issues with
64-bit unsigned types and signed ints. Recasting it explicitly
to use a ptrdiff_t type fixes the problem:

    c++;
    if ( (ptrdiff_t)(c - retbuf) >= BUFLENG ) {

Before this fix pairndf was failing even before it started with
a buffer overflow in the Tcl communication system.

After this patch pairndf at least throws up a couple of images
although I'm not sure what I'm supposed to do with them once
they are up.