filefec: Bug fix for the files with certain lengths (see the longer explanation)

Bodecs Bela (bodecsb@vivanet.hu) reported cases where files with
certain sizes, when encoded and then decoded produces slightly longer
files with some extra padded bytes. To reproduce the bug, the input file
sizes needs to be any length in the range:

 (n * 4096 - (k - 1)) < length <= (n * 4096)

Thanks to Bodecs and jg71 for bringing it to my attention.
(https://tahoe-lafs.org/pipermail/tahoe-dev/2013-September/008699.html)