Fix decoding of base64. We got the length wrong when the result was not

Fix decoding of base64. We got the length wrong when the result was not
an exact multiple of 3.

I also wrote a torture test and it survived some minutes of random stuff
coded/decoded up to 16 MB data. But that would be a bit too embarassing to
commit... :-)

Volker
(This used to be commit 3fda2a0432a1dc7a0d28d83e35f6329bc30378e0)