Fix UB in cups_raster_read debug message

Fix UB in cups_raster_read debug message

The first time `cups_raster_read` is called, both `r->bufptr` and
`r->buffer` are NULL.  The calculation here then ends up looking like
adding a size_t to a NULL pointer, which triggers the ubsan detector.
Since we just want an offset, cast the pointer difference to ssize_t
like the code already does a few lines below.