Fixed bug in bucket shakedown code: it crashed with a mysterious error message

Fixed bug in bucket shakedown code: it crashed with a mysterious error message
("Unexpected EOF") when there was a bucket larger than the shakedown buffer
and this included even deleted buckets (which was the cause of the latest crash
on sherlock5: shakedown on a corrupted database patched the corruption by
a large deleted bucket and when I ran it again, it crashed again due to this
bucket).

Now we are able to cope with deleted buckets of any size and when we
encounter an oversized non-deleted bucket, we bail out with a proper
error message.