Fix bug #6 and #7

Instead of using an offset and nmemb in write_fasplit I decided to
use a simpler solution which fixes the problem. Before a part of a
chunk is to be written to a separate file that part is copied as
a substring to a separate buffer. Then the write_fasplit takes that
whole substring and writes everything.

This fixes the problem and it could be enhanced later in the future.
See enhancement #11 as a reminder how this could be improved.