Cryptbucket check nonce length

[ArneBab]

addresses https://freenet.mantishub.io/view.php?id=6363

based on the patch therein. For review and testing.

[nextgens]

AEADOutputStream talks about nonce[0] &= 0x7F but doesn't enforce it in its constructor; IMHO we want/need both

[nextgens]

Let me be more clear: either we try to "fix" what we get (masking the bit we discard) or we enforce that we never create a bucket with it set... right now we don't...

My preferred solution is to mask the bit rather than throw (to keep backward compatibility)

[ArneBab]

@nextgens could you file a new pull-request which does what you prefer?

[nextgens]

I will try to have a look this week-end

[ArneBab]

Thank you!