Fix the cmyk function for the color black

[Josef-Friedrich]

For the color black, the variable K is 0. Lua interprets 0 / 0 as -nan

lua-color/init.lua

Lines 540 to 548 in eba73e5

	function Color:cmyk()
	local r, g, b = self.r, self.g, self.b
	local K = math.max(r, g, b)
	local k = 1 - K
	local c = (K - r) / K
	local m = (K - g) / K
	local y = (K - b) / K
	return c, m, y, k
	end

color = Color('#ffffff')
print(color:cmyk()) -- -nan	-nan	-nan	1.0