block: NULL dereference on error path in __blkdev_get()

"disk" is always NULL when we goto out.  There was a check for this
before, but it was removed in 69e02c59 "block: Don't check events
while open is in progress".

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@carl>