Fix corner case in bad block table handling.

In the unlikely event that both blocks 10 and 11 are marked as bad (on a
32 bit machine), then the process of marking block 10 as bad stomps on
cached entry for block 11.  There are (of course) other examples.

Signed-off-by: Patrick Doyle <pdoyle@irobot.com>
Reviewed-by: Richard Weinberger <richard@nod.at>