always initialize first and last

With CFLAG -O and -Wall this gives a "maybe uninitialized" warning:
```
gcc -c -std=c99 -pthread -I/usr/include/libmirage -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include  -g -Wall -O  image_id.c -o image_id.o
image_id.c: In function 'process_disc':
image_id.c:200:2: warning: 'last' may be used uninitialized in this function [-Wmaybe-uninitialized]
  for (int i = first; i <= last; i++) {
  ^
image_id.c:205:12: warning: 'first' may be used uninitialized in this function [-Wmaybe-uninitialized]
  discid_put(discid, first, last, offsets);
            ^
```