avfilter/dctdnoiz: fix slice_h computation

ceilf() can only work if the reminder of the division is not 0.

This fixes memory errors with for instance:
  ffmpeg -f lavfi -i testsrc=s=800x500 -threads 3 -vf dctdnoiz -frames:v 1 -f null -

(cherry picked from commit eb7efaa9)