Division by zero  #5

In ColorHelper/ColorHelper/Converter/ColorConverter.cs,
at lines 23 to 25:
c = (1 - modifiedR - k) / (1 - k);
m = (1 - modifiedG - k) / (1 - k);
y = (1 - modifiedB - k) / (1 - k);
we have a division by zero if k == 1. k is computed at line 22 as one minus the max of r,g,b.
So if all of r, g, b are 0, the max is zero, then k is 1, and 1-k is zero. Division by zero.

It seems that in CMYK, if K is 1, the values of CMY don't matter, so we should set them explicitly to 0.