libblkid: fix segfault in blkid_do_probe()

libblkid: fix segfault in blkid_do_probe()

This:
	pr->cur_chain += sizeof(struct blkid_chain);

is nonsense of course, there should be a cast to (char *) or so.

It seems that the most robust solution is to avoid this game with
pointers and use chain->driver-id which is useful as array index.

Signed-off-by: Karel Zak <kzak@redhat.com>