update: Fix sum of credits for FX
update: Fix sum of credits for FX

* Fixed sum of credits for FX. The assumption is that when someone fails
  a class (and thus gets an FX) the number of credits recieved for such
  a class should be zero (0).

* Fixes #67.

Signed-off-by: mr.Shu <8e7be411ad89ade93d144531f3925d0bb4011004@shu.io>

fmfi-svt/votr,fmfi-svt/votr