We need to check for a valid index before accessing the array
element to avoid accessing invalid memory regions.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>

Modified by Jens to drop the unlikely(), and make the fall through
path be having a valid tag.
Signed-off-by: NJens Axboe <axboe@fb.com>